Most games not launching

Hello People. Until recently on my old(!) computer I have been using Manjaro but now I have a new computer, all the hardware Linux compatible, and on that I installed Pop_OS! 19.10 because I expected it to work better with Steam as they are more closely related.
Sadly, since the change, many games that used to work fine work no longer. The problem specifically is that many won't start properly and this affects a whitelisted game too.
so, here are the various errors that occur:
Steam gets stuck in "preparing to launch game" forever.
Steam lables the game as running, though it's not running. (most common)
The Play button turns blue for a few seconds and then green again as if I never clicked it.
The Game launches but shows nothing but a black screen.

My steam library is located on a second internal ssd on an ntfs-formatted partition. I moved some games to ~/.local/steam, but only saw changes in one of them and that one closed after a few minutes of playing (Dark Souls III). I have gone over the requirements for proton and tried multiple proton versions. But I am certainly no pro and probably haven't thought of all the things one might try out.

I know that I am not really providing any helpful data. That is because I don't know which data would be helpful. I'll happily provide anything that might help to get those games running again.
I thank you all for any recommendations and hints you might have.

Distro: Pop_OS! 19.10
Graphics Card: NVidia GeForce RTX 2060 with driver 440.44
Proton version: any
Tested whitelisted games without problems: The Room, South Park Stick of Truth
Tested non-whitelisted games without problems: Risk of Rain 2
Tested whitelisted games with problems: Dark Souls III
Tested not whitelisted games: Frostpunk, Brawlhalla, Hob, Resident Evil Biohazard HD Remake, Digiom Story Cyber Sluth, Final Frantasy X|X2 HD Remaster, articy:draft Yu-Gi-Oh! Duel Links

No source file, test, or entry point is identified. Start by reproducing the launch failures using the listed Pop!_OS version, NVIDIA driver, Proton versions, NTFS library, and affected games, then collect the missing diagnostics; done requires isolating a specific reproducible failure.