In my case, it didn't showed any error, the game simply did not do anything.
I had to launch proton with the launch option PROTON_LOG=1 %command%
In the logs, the error comes from wine, telling that a file was not owned by me.
I was installing a game in a NTFS partition mounted by root (so owned by root), with all permissions open (777)
The problem went away when I mounted the disk with my user, but it was very hard to find the problem, a normal user would have give up, I spent a couple days thinking the game was corrupted and redownloading more than 30Gb
PROTON_LOG=1x1 2019-09
'Proton: Permission denied" error launching shared Proton games but only for the user who don't own the game. It's maybe because Proton use a different folder for his things?
I have a shared folder for multiple users (with ACL), it works perfectly, the only problem is with Proton games, and only for the user who don't own the games.