Black Ops 1 PS5 Zombies “Unable to Join Game Session” — Here’s What Actually Works

Trying to play Black Ops 1 Zombies on PS5, but matchmaking keeps searching, fails to find players, and eventually shows “Unable to join game session in progress”?

You are not necessarily doing anything wrong.

Since the PS5 version launched, players have reported problems finding public Zombies lobbies, with some maps appearing much harder to matchmake on than others. Kino der Toten and Ascension are frequently mentioned, while other maps may work after several attempts.

The important thing is to determine whether you are dealing with a matchmaking problem, a connection problem, or a temporary lobby issue before changing random router settings.

Here is the troubleshooting process worth trying.

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What the “Unable to Join Game Session in Progress” Error Means

When you search for a public Zombies match, Black Ops 1 tries to place you into an available lobby.

If matchmaking repeatedly finds a session that you cannot enter, you may see:

«Unable to join game session in progress»

The game can then place you into your own lobby, where you wait for other players but nobody joins.

That does not automatically mean that Black Ops 1 Zombies is dead or that your internet connection is broken.

If multiplayer works normally while Zombies matchmaking fails, that is an important clue.

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1. First Check Whether Multiplayer Works

Before changing anything on your router, test regular Black Ops 1 multiplayer.

If multiplayer works but Zombies doesn't

This points more toward a Zombies matchmaking problem than a completely broken internet connection.

If both multiplayer and Zombies fail

Then your connection becomes more suspicious.

In that situation, check:

- PS5 internet connection

- NAT type

- Router connection

- UPnP

- Possible ISP/CGNAT problems

- Activision service status

Don't start changing advanced settings just because one Zombies map refuses to find a lobby.

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2. Completely Exit Black Ops 1 and Restart It

This sounds too simple, but it is worth trying.

Don't just leave the Zombies lobby.

Do this:

1. Exit Black Ops 1 completely.

2. Return to the PS5 home screen.

3. Close the game.

4. Start Black Ops 1 again.

5. Enter Zombies.

6. Search for a public match again.

Some players have reported that returning all the way to the main menu and reloading Zombies allowed matchmaking to work again.

This is one of the first things to try because it costs nothing and doesn't change your network configuration.

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3. Don't Assume Every Zombies Map Has the Same Matchmaking

This is one of the most important tests.

If Kino der Toten refuses to find a lobby, try another map.

For example:

- Kino der Toten

- Ascension

- Der Riese

- Nacht der Untoten

- Moon

- Five

Recent players have reported being unable to find matches on certain maps while successfully finding games on others.

That means repeatedly searching the same map for 20 minutes may not tell you anything about your internet connection.

Try this instead:

Map A fails → try Map B → compare the result.

If another map immediately finds players, your connection may not be the main problem.

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4. If You Get Put Into an Empty Lobby, Search Again

One reported workaround is to avoid simply sitting in the empty lobby forever.

If the game repeatedly:

1. Searches for players

2. Says “Unable to join game session in progress”

3. Places you into your own lobby

try searching again rather than assuming the session is completely broken.

Some players report needing multiple matchmaking cycles before getting into a working lobby.

It is frustrating, but the behavior appears to be inconsistent.

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5. Try a Different Time — But Don't Assume It's Just a Player-Count Problem

You might think:

«“Nobody is playing Zombies.”»

That's possible in some regions or at certain times, but recent player reports make that explanation too simple.

Players have reported the same matchmaking behavior even when attempting to play at different times and on popular Zombies maps.

So if you have been searching for several minutes and repeatedly receive the same error, don't immediately conclude that BO1 has no players.

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6. Try Joining a Friend Instead of Public Matchmaking

If public matchmaking isn't working, test a private friend connection.

Have one player:

1. Start Black Ops 1.

2. Enter the appropriate Zombies lobby.

3. Stay in the lobby.

4. Have the other player attempt to join.

If normal public matchmaking fails but joining a friend works, that's useful information.

It suggests the problem may be related to how the public matchmaking system is finding and connecting players rather than your PS5 being completely unable to communicate with the game.

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7. PS5 Friend-Join Workaround Worth Testing

Some players have reported a different approach that can help with BO1 PS5 party joining.

The reported method is:

1. Have the party leader open the game and remain in the lobby.

2. The other player completely closes Black Ops 1.

3. From the PS5 home screen, open the friend's PSN profile.

4. Use the available Join Game/Join Session option.

5. Allow the PS5 to launch Black Ops 1 and attempt to place you into the friend's lobby.

This is not a guaranteed official fix, but it is worth testing if normal invitations repeatedly fail.

If it works for you, it can be much faster than repeatedly accepting an in-game invitation that does nothing.

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8. Check Your NAT Type — But Only If Other Online Problems Exist

If you cannot join friends, public matches fail across multiple modes, or Black Ops 1 consistently struggles to connect, check your PS5 NAT type.

On PS5:

Settings → Network → Connection Status → View Connection Status

Look for your NAT Type.

A normal PS5 internet connection can still have NAT-related compatibility problems with older multiplayer games.

However, don't assume:

«“My PS5 says NAT Type 2, therefore BO1 should have no connection problems.”»

Older games can behave differently from modern multiplayer titles.

That's why the most useful test is comparing:

BO1 Zombies + BO1 Multiplayer + another online game.

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9. Check Whether Your Router Uses UPnP

If both Zombies and multiplayer are having connection problems, check your router's UPnP configuration.

UPnP allows devices such as your PS5 to request the ports they need automatically.

If UPnP is malfunctioning, older games can sometimes behave strangely.

However, don't randomly change UPnP settings if everything else works.

If Black Ops 1 multiplayer works perfectly and only Zombies matchmaking is failing, changing your entire router configuration could create more problems than it solves.

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10. Don't Immediately Blame Your Internet Speed

A fast internet connection does not automatically guarantee perfect matchmaking.

You could have:

- 500 Mbps

- 1 Gbps

- Fiber internet

- Low latency

and still experience matchmaking problems.

The important factors include NAT compatibility, routing, host connectivity, matchmaking behavior and the game's online infrastructure.

So running a speed test and seeing 900 Mbps doesn't prove that BO1's matchmaking should work perfectly.

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11. Test With a Different Connection

This is one of the best diagnostic tests if you suspect your home network.

If possible, temporarily connect the PS5 through a phone hotspot or another internet connection.

Then test Black Ops 1 Zombies.

If BO1 works on the alternate connection

Your home network or ISP becomes more suspicious.

If BO1 behaves exactly the same

The problem is more likely related to the game, matchmaking, account/session behavior or the specific lobby/map.

This test can save you from spending hours changing router settings for no reason.

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12. What About CGNAT?

CGNAT can cause problems with peer-to-peer multiplayer games because your router may not have a directly reachable public IPv4 address.

A common CGNAT range is:

100.64.0.0 – 100.127.255.255

If your router's WAN/Internet IPv4 address falls inside this range, ask your ISP whether your connection is behind CGNAT.

You can ask:

«“Is my internet connection behind CGNAT? My router is receiving a 100.x.x.x WAN IPv4 address. Can you provide me with a public IPv4 address or remove CGNAT from my connection?”»

However, don't assume CGNAT is the reason for every BO1 Zombies matchmaking problem.

If other online games work and BO1 multiplayer works, while Zombies alone refuses to find a lobby, the evidence points somewhere else.

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13. Don't Waste Time Uninstalling the Game First

If Black Ops 1 launches correctly and you can play other online modes, reinstalling the entire game should not be your first move.

Try the diagnostic steps first:

Restart → different map → different matchmaking attempt → friend join → NAT check → alternate connection.

Only consider reinstalling if you are experiencing broader installation, crashing or corrupted-data problems.

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The 5-Minute BO1 Zombies Test

If you just want the fastest possible diagnosis, do this:

Test 1

Can you play Black Ops 1 multiplayer?

Yes → your basic connection to BO1 is working.

Test 2

Can you find a Zombies lobby on another map?

Yes → the problem may be map/matchmaking specific.

Test 3

Does completely restarting BO1 change anything?

Yes → you may be dealing with a temporary session/matchmaking problem.

Test 4

Can you join a friend?

Yes → public matchmaking may be the problem.

Test 5

Does BO1 work through another internet connection?

Yes → investigate your home network or ISP.

Test 6

Do multiple BO1 online modes fail?

Yes → now investigate NAT, UPnP, IPv4 and other network issues.

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So, Is Black Ops 1 Zombies Broken on PS5?

The honest answer is:

There are enough recent player reports to show that Zombies matchmaking on the PS5 version can behave badly, but there isn't one universal fix that solves every case.

Players have reported:

- “Unable to join game session in progress”

- Empty Zombies lobbies

- Repeated failed matchmaking

- Certain maps being harder to matchmake

- Difficulty joining friends

- Multiplayer working while Zombies doesn't

- Success after restarting or repeatedly searching

- Different results depending on network configuration

That means the smartest approach is diagnosis instead of blindly changing settings.

If only one map refuses to matchmake while other online modes work, don't immediately blame your router.

If every BO1 online mode fails, investigate your connection.

And if another internet connection suddenly makes everything work, you have a much stronger case that your home network or ISP is involved.

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Final Fix Checklist

Before giving up on BO1 Zombies, try these in order:

✓ Completely close Black Ops 1

✓ Relaunch the game

✓ Try another Zombies map

✓ Search again after the failed session

✓ Test Black Ops 1 multiplayer

✓ Try joining a friend directly

✓ Test the PS5 friend-list Join Session method

✓ Check PS5 NAT Type

✓ Test another internet connection

✓ Investigate UPnP/NAT/CGNAT only if multiple online modes are affected

✓ Check Activision service status if the problem suddenly affects everyone

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The Bottom Line

If Black Ops 1 Zombies on PS5 keeps saying “Unable to join game session in progress,” don't assume your PS5, internet or the Zombies player base is automatically the problem.

The fastest way to find the cause is to compare different maps, different modes, friend joining and different network connections.

If multiplayer works but Zombies doesn't, focus on matchmaking first.

If one Zombies map works while another doesn't, don't start rebuilding your router configuration.

And if nothing works across multiple online modes, then start investigating NAT, UPnP, IPv4 and ISP-level connectivity.

That's the difference between randomly trying fixes and actually finding out what's wrong.