Notes:
1. I encountered this issue on Windows 10/ steam
2. I was playing FRIENDS
3. I was playing as Archer

Steps:
1. friend started session friend only session

Result:
one of the following happened
1. joined my friends game but they where not there
2. tried again came up with session is in progress
3. sometimes came up with my character is already in the game (not true they are in the same house)

restarted game on both machines multiple times does not seem to fix issue.
It seems like something is persisted on the network and is not being cleared