Unable to host/join online games

Hi, I’m getting the following very cryptic and unhelpful error message when I try to join online lobbies:

"This game is invalid. The host is not in his own room"

Which to me sounds impossible and paradoxical.
Alternately, people trying to join me will get the same message when I host.

Have thus far tried (without success):
Verifying game files
Removing mods
Restarting the computer

I’ve had no trouble up until now, and nothing has changed on my computer recently so I’m really confused as to how this issue could just pop-up out of nowhere.

Thanks in advance for any help.