"Unable to start the session" multiplayer launch failure.

[MadScientist212]

I've gotten this message a ton of times when attempting to start a new multiplayer game with my brother or brother-in-law. The session is initially hosted fine, no apparent issues at first. We set factions and game options and hit start, the map gets generated, progress bar fills, it reaches "uploading", then it kicks back to the session menu with "Unable to start the session." This has happened multiple times with both me and my brother attempting to host and start the game. No details given, no error message aside from "unable to start the session". Attempting to start again usually fails, but sometimes creating a brand new session the game will start fine. No difference in options or settings between when it starts the game fine and when it kicks back to the session menu could be determined.

I've looked around and I've seen a lot of posts about multiplayer issues but can't find any with this specific message or issue. Anyone else getting it?

[MadScientist212]

"Unable to launch the session"
"Unable to launch the session"
"Unable to launch the session"
"Unable to launch the session"
"Unable to launch the session"
"Unable to launch the session"
"Unable to launch the session"

Nothing like having your previous game become unplayable because of sync errors that reverting the session fails to fix (after a previous error forced you to revert all the way back to turn 4) and then spending an hour trying to start a new game, only for it to be impossible because every single time you try, literally every time:

"Unable to launch the session"
"Unable to launch the session"
"Unable to launch the session"
"Unable to launch the session"
"Unable to launch the session"
"Unable to launch the session"
"Unable to launch the session"

What is the deal with this?