I am the friend that was launching the game on my PC that @molten dagger mentioned in the comment above. I have no idea how to report any issues because I'm not getting any feedback or visibility in how to fix this.
Everything seems to launch per usual for me, no issues, no complaints, no hints at anything going awry.
@molten dagger was never able to find my game last night and I tried multiple save files. One from the night prior we had been playing successfully with a friend. One from a few days prior where it was just her and I.
We are trying again right now but she was unable to join both of those saves I mentioned before. I actually tried to invite her to a "co-op" game that I had been playing solo, never invited anyone before, so I could see if the "invite" feature would work and she was never able to find the game - no matter what we tried.
I tried seeing if I could get her to host a game for me to find and she got the same error screen others are getting. @molten dagger has now uninstalled and is re-installing the game now.
We both play through Steam. Last night I checked to see if I had to the most recent version and I did.
We have our other friend hopping on now to see if she can see any of the hosted games