We are reciving reports of players attempting to play multiplayer on Steam but are getting a connection error. Our team is currently investigating this issue, however we have found a workaround in the meantime.
It has been reported that if you remove the people who you are trying to play with from your Steam Friends List and join using Direct Connect, it should work.
Please keep in mind as well that if one person has mods installed, all must have the exact same mods installed - no more, no less. It is also important to double check that everyone is using the same mod version.
If you have tried this and are still unable to join, please don't hesitate to reach out ot our Support team:
https://larian.com/support/faqs/how-to-submit-a-bug-report_85