Try everyone exiting out of Steam/Galaxy, create a new Windows user account, switch to that account, start the game from the executable, create a new profile when prompted and try testing with an included campaign, joining by Direct Connection. If that helps, the problem is likely in the mods or profiles on one or more systems, which testing in pairs (or other people) could help identify.
If the same thing happens in new Windows user accounts, the issue is likely in the install folder of one or more systems.
To check for mismatched data, if you each use a program like DVDSig or ExactFile to scan the install folder, then swap files to verify, it would identify any differences. There would be legitimate differences just from having the editor installed on at least one system, but if you move/delete any mods or projects you are not using first, hopefully it will identify what is causing the problem.
DVDSig is tiny (
alternate download), portable and doesn't require an install, while
ExactFile has more features.
