Same issue here. EndevaourOS- Proton Experimental Bleeding Edge and Proton Experimental. Proton 9 works fine.
Hello @Leopard1907, we're using one issue report per unofficially supported game title, so I've gone ahead and transferred this issue report to https://github.com/ValveSoftware/Proton/issues/3916#issuecomment-3393791193.
proton experimentalx2 2025-10proton 10.0-2x1 2025-10proton 9.0-4x1 2025-10
Compatibility Report
steam-6510.log
System Information
I confirm:
Symptoms
The game launches, displays the Capcom logo, and then shows the error message:
“Steam is not running. Please start Steam then start the game.”
Steam is running on system.
The issue occurs with Proton Experimental, Proton 10.0-2 (beta) (as of October 2025).
However, it works with Proton 9.0-4, which means that there is a regression
Reproduction
Launch Steam.
Set Lost Planet: Extreme Condition (AppID 6510) to use Proton Experimental.
Start the game.
After the Capcom intro, observe the “Steam is not running” error dialog.