This is mostly a potential fix for you. We have contacted Steam about this problem since it popped out of nowhere into our game and we're sure it's mainly an issue with Steam's servers and Panicore being affected by some limitations. The following steps have helped players in our Discord server, if they don't work feel free to enter our Discord and we can try finding a fix together!

EDIT 1: We're rewriting the whole lobby system to get rid of this problem. The system is in testing, but we had some progress with it, so feel free to test it out! Switch to "new_lobby_beta" in the Beta tab of the game's properties.

• you join your friend's lobby via invitation, but it kicks you back to the main menu
  
• you join your friend's lobby via the session browser, but it kicks you back to the main menu

• Change the Download Region in Steam's settings
  
• Try setting the region to the same as your friend's
  
• EDIT 1: Try beta version "new_lobby_beta", it is still in testing, but might work out