Slitherine

The modder "BlackArchon" created a mod with additional factions.
We wanted to test his mod. I, at first, installed the mod in the wrong folder.
When I clicked on Multiplayer=>Game Browser, the game crashed with a message that files of the "Gaians", a faction from BlackArchons Mod, was missing.
After installing the Mod correctly, I could see his game.

This means if someone hosts an MP game with this or other faction-adding-mods, Multiplayer is essentially shut down for everyone who doesn't have the Mod installed.

I was there too. We came to the conclusion that it most likely has to do something with the displaying of the factions ingame when you hover over the game name in the MP lobby. If there are factions displayed that your game doesn't know it shuts down. At least that's our theory.

Exactly. Instead of trying to show these factions, in this case the game should be greyed out or so and a hint should appear, that it is ran with a mod, that is incompatible to the players version.
If the mod gains some popularity, this really could become a problem that has to be dealt with.
Right now using the beta might be a good-enough workaround.