Before i begin, i have purchased BO2 For steam and i have 3 copies installed.
This worked on Redacted LAN and i have not tested this on steam.

If you use Redacted make sure the STEAM account is the same throughout.
For example, i have 3 steam accounts, my third has my BO2 stats attached.
Because i logged into my stats on a different steam account, i got the server error.
I started to get annoyed because i had already prestige 2 and didn't want to start over.

when i tabbed out, i realized that i was logged into my main steam account. When i switched to my third account, the error went away and i was able to play the game again.

So make sure your signed into the correct account and you dont edit your stats and it should be good!