I'd suggest deleting proton experimental, it might have gotten corrupted and steam is choking on it for some reason. You should be able to delete it from Steam in game mode, or you can install Protonup-qt from the discover store and delete it that way.
For games there are also appmanifest files in the steam folders that steam uses to identify what's installed. If uninstalling proton experimental doesn't help, it may be that there's a corrupted manifest file (or something similar) for it that needs to be manually deleted. Without doing some digging on my own computer I couldn't tell you what the file name/location is, and I won't be able to check my PC until later today or tomorrow.
can you provide more information ? like what are you trying to do ? if this happen while launching steam itself or something else? are you trying to launch a linux binary with proton ?
Just launching Steam
