Hello @dominikzogg, this was previously discussed at #2459. Please go to Steam -> Library dropdown -> Tools and install Proton 4.2 in the list.
"Fixed an issue where the Steam Client wouldn’t automatically update or download Proton before launching games through Steam Play" in the 2019-03-28 Steam client beta update should have resolved this issue. Please opt into Steam's beta client.
proton 4.2x1 2019-03
Games like GTA V and Shadow of a Tomb Raider worked very well on (3.16-8). Some known glitches. But after i've upgraded Proton to 4.2.1 within Steam no Game starts. Downgrade solves the issue