Hello @robschi, this sounds like #3528, you can give https://github.com/ValveSoftware/Proton/issues/3528#issuecomment-589828273 a try.
Thank you for the reply kisak.
I tried the mentioned workaround in bug #3528.
I started the game with PROTON_DUMP_DEBUG_COMMANDS=1 %command%, executed /tmp/proton_$USER/run regedit, added the mentioned registry key, and added the string value "UseXIM" with the date "y" under it and restarted the game, but this did not solve my problem.
And even if this has solved the problem for the game with which I had tested it, I would have to do this for every game.
The bug #3528 was never really solved, only a workaround was described in it, and this workaround could not solve my problem.
PROTON_DUMP_DEBUG_COMMANDS=1x1 2020-03
Proton version: 5.0-5
In games I normally use the 'right Alt' (Alt Gr) key for functions in games.
But in games which use Proton it's not possible to assign this key.
Since it works with native Linux games, I assume this is a problem with proton.
(I have a German keyboard layout.)