Multiplayer version checksum mismatch

It sounds like one of you is running a mod, or editing your game files. If you're running mods, all players in a lobby have to be running mods in order to connect to each other.

It could also be a hard drive issue (eg. your hard drive is failing), or you have a virus that's editing game files.

You can go to support.paradoxplaza.com and open a ticket if you need further support with this issue.

Hope this helps!