https://bugs.winehq.org/show_bug.cgi?id=52113
Gijs Vermeulen gijsvrm@gmail.com changed:
What |Removed |Added ---------------------------------------------------------------------------- Severity|major |normal Summary|Killing Floor 2 : Item |Killing Floor 2 item server |Server not connectable |can't be reached
--- Comment #1 from Gijs Vermeulen gijsvrm@gmail.com --- Using Proton is not supported, not the official version nor the custom GE version. That's because they both include a bunch of custom patches and it would be very hard to rule out if any of those patches cause this issue.
So unless you are able to install and run the game with upstream wine, I think this bug report is invalid on this bug tracker.