UPDATE:
Earlier some friends and I were all in a lobby together when we decided to get perma-jug. In order to get it, we went into our own solo lobbies and used the "fast_restart" command to get perma-jug faster. After we did this, we couldn't join each other and got the "Unable to join game session" error again.
It could just be a coincidence, but we were using console commands both times. The first time (referenced in the original post in this thread) we used "god" and the second time (this time) we used "fast_restart"
Once again, this all could be a coincidence but I think maybe using the console commands has something to do with it.