DayZ: attempting to connect to servers with Proton Experimental/9.0 returns "Connecting failed (0x00020009)"

[aclist]

• Steam client version (build number or date): 1708985249
• Distribution (e.g. Ubuntu): Arch Linux
• Opted into Steam client beta?: Yes
• Have you checked for system updates?: Yes
• GPU: Nvidia

Steps for reproducing this issue:

This appears to be a regression introduced in the new version of Proton.

1. Set DayZ's compatibility tool to Proton Experimental or Proton 9.0 Beta
2. Launch DayZ
3. Select Play DayZ if prompted in the launch options dialog
4. Use the quick server launcher or click Play to boot directly into the main menu
5. Select any server (happens on any; for this example, use an Official server as a control)
6. Click to connect and wait
7. Returns an error dialog: Connecting failed (0x00020009)

The problem is resolved by rolling back to Proton 8.05.

[kisak-valve]

Hello @aclist, we're tracking issues with games run with Proton in the Proton issue tracker. Additionally, 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/3899#issuecomment-1968096123.