Proton broken?

So, I'm not really sure where to ask properly. So, let's try my luck here :)

A couple of weeks ago, a bunch of games I had been playing on Steam through Proton just abruptly stopped working. I got some to work by adding PROTON_USE_WINED3D=1 %command% in each broken game's launch options. Therefore it has something to do with OpenGL, if I understand correctly.

Some games that were affected were Fallout 3 and Mafia. Now Mafia seems to run without that launch option again, but Fallout 3 still doesn't even launch without it.

If I set that launch option, I get as far as this menu:

...but clicking on Continue or Loading a save crashes the game immediately.

I've been playing Fallout 3 on this machine for over 10 hours just fine, and this problem just appeared one day. Anybody know (or could guess?) what the root cause is and what could be done about it?