Hello @deathromantik, do you happen to have multiple Steam libraries, and if you do, what filesystems are they on?
Download and validation of game files is managed by the steam client, not by Proton, so I've transferred this issue to the steam-for-linux issue tracker.
Hello @deathromantik, do you happen to have multiple Steam libraries, and if you do, what filesystems are they on?
My system has nothing exotic for a Linux Steam install. Just one default steam library on an ext4 file system.
It doesn't seem to affect native Linux titles but it does seem to affect when I force some to use Proton.
I have a sinilar problem with Killing Floor 2 where it cannot properly download the game and I see this when I point the cursor.

So, I installed the game in separate steam instance in wine, made a backup and restored it in linux client.
Nothing extracted yet.
There is a problem with Borderlands TPS when you opt to force use of the proton to use the Windows version with the latest Steam beta as of 20th of January 2019.
Files are downloading corrupted, no mater how many times I delete the affected file and validate/redownload said file.
On launching I get the following error:
An error occured while updating Borderlands: The Pre-Sequel (corrupt content files): /home/USER/.steam/steam/steamapp/common/BorderlandsPreSequel/steamassets/dlc/marigold/lic/audio/english(us)/audio_streaming_marigold.pck See the Steam support site for more informationBorderlands 2 has a similar issue, I am trying to track down which file is causing the issue for that game and will open another issue for that game.