hima123 Yo boys, I had this problem as well. Apparantly I had not installed DLCs or I made a mistake when merging dlcs with the main game. Delete everything and reinstall. Should work.
Actually I confirmed this by going to server with different zombie maps. Town and farm were working fine, but this error would come up with mob of the dead and nuke.

Alexei
@Alexei
Posts
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i keep getting this message (An error occured while connecting to the server) i cant join any bo2 zombie server i tried everything -
You are connected to the server bugXerxes
Hello staff- Its never me who disconnects the server, Its the server who disconnects me, so I cant really help that disconnection. Also, When I tried to join the server for the very first time ever, It did the same thing I mentioned above.
- My buddy who was able to join the server waited till he got the "playername timed out" message to appear, so that I can try to join after that. I tried, I couldnt, same error message. And this same thing is occuring in many different servers.
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You are connected to the server bugWhen I play some server for 2-3 matches, and then when the 4th match starts, I get randomly disconnected from the server with the error message "An error occured while trying to join the server", and when I try to join back, it says "You are already connected to the server".
My buddy who is in the same server says that he sees my name in the tab list with a disconnect symbol next to my name.
Any idea how to fix it?
Thanks in advance.