The problem is less frequent when running with the original Steam runtime, so I guess this is a problem with the librairies I use.
Sorry for the inconvenience, closing the issue.
(You might want to look at the source-1-games issue tracker, with bigger issues.)
Nothing extracted yet.
This is the error encountered when restarting Steam or starting it automatically (here using MATE).
I use Latest Steam Beta, Arch x64.
System specs here.
I can provide Archey's output if needed.