SCP SL repeated kicks from servers

[Magnopolis]

Compatibility Report

• Name of the game with compatibility issues: SCP: Secret Laboratory
• Steam AppID of the game: 700330

System Information

• GPU: AMD Radeon RX 9060 XT
• Video driver version: Mesa 26.1.2-arch1.1
• Kernel version: Linux 6.18.35-1-lts
• Link to full system information report as Gist: https://gist.github.com/Magnopolis/424907072ef04b89a3844a1f8969c9bd
• Proton version: GE10-34 and experimental

I confirm:

• that I haven't found an existing compatibility report for this game.
• that I have checked whether there are updates for my system available.

steam-700330.log,
scpsl_filtered_experimental.log (filtered, because original log was 2 GiB)
steam-700330 proton GE 10-34.zip

Symptoms

I join a Server, i play and at first everything goes smooth. Then in the middle of a round (not while a new round begins or i get spawned) i get kicked with the message "YOU HAVE BEEN KICKED The server was forced to stop the connection". That happens every time eventually. No proton version fixes it, switching from the linux kernel to linux-lts didn't help, installing missing lib32 files i forgot to download didn't help. Using Steams flatpack version didn't help too.
The Proton log shows "manual_convert_Set_SteamAPI_CCheckCallbackRegisteredInProcess_func_156 not implemented!" firing twice immediately after SteamAPI init. No localAuthToken.txt is ever generated, meaning Northwood's central auth server never successfully verifies the Steam identity.

Reproduction

The same game, same Proton versions, works fine on EndeavourOS. The issue is specific to Arch Linux i think.

[kisak-valve]

Hello @Magnopolis, we're using one issue report per unofficially supported game title, so I've gone ahead and transferred this issue report to #4229 (comment).