Possible fixes for an issue where players try connecting to a server just for it to lose connection.
How to Fix “Lost Connection to Host” Error
I personally have found that trying multiple times to connect to a server eventually works. Sometimes it works after 3 tries, sometimes 5, sometimes once. It’s really unpredictable, and it has nothing to do with how many players are on the server or playing Black Ops as a whole.
Although this is really generic, your anti-virus or firewall could be messing with the game. Make sure both Private and Public connections are selected on Black Ops – Multiplayer in Windows Defender Firewall.
Try flushing your DNS
Open Windows Start
Type cmd, and run it as administrator
This has been reported to work for some users, so I highly recommend trying this.