Bug/Issue Me and my partner suddenly unable to join each other through steam?

Me and my partner play together by joining each other's games through Steam.

That worked fine for months, all the way until last night.

Today, however, we have tried joining each other's games, but either of us doing it just gives us the message "Join failed! Timeout while establishing connection."

We have not installed any new mods. I don't think either of us have also had any updates to our computers. We just suddenly can't join each other's games because of "timeout" and we literally have no idea what the issue could be.

I have tried searching for solutions but all I have found are other unanswered threads asking for help with a similar issue.

Help??

 

Got the same problem, ive tried everything too, from firewall allowance to port forwarding, reinstalling the game, reinstalling steam, reseting my router's settings, and running the game as an administrator. It started happening ever since the 1.03 update