Here's what happened:
- Me and my friend were playing with a drop-in skater in our Club 3vs3.
- After the game, the drop-in skater was showing as "in post-game".
- We left the Club 3vs3 lobby to get rid of the drop-in skater.
- When trying to come back to Club 3vs3, we got "Invalid Game Session".
- We restarted the game, still the same issue.
- We were able to get into a Club 6vs6 lobby and played with only computer teammates.
- We disconnected from the game. Tried to get back to Club 3vs3, got the "Invalid Game Session".
- We went to play a Quickplay 3vs3 with a drop-in skater.
- We completed the game.
- We were able to enter our Club 3vs3 lobby, no more Invalid game session.
Hope it helps.