Compatibility Report

• Name of the game with compatibility issues: Somnium Space / Somnium Space VR
• Steam AppID of the game: 875480

Note: this was reproduced with the Somnium Space Launcher executable added as a non-Steam game. Steam assigned the local run SteamGameId: 15466454116586225664.

System Information

• GPU: Intel Raptor Lake-S UHD Graphics + NVIDIA AD106M [GeForce RTX 4070 Max-Q / Mobile]
• Video driver version:
  • nvidia-utils 610.43.02-3
  • lib32-nvidia-utils 610.43.02-1
  • mesa 26.1.1-1
  • lib32-mesa 26.1.1-1
• Kernel version: 7.0.10-2-cachyos
• Link to full system information report as Gist: https://gist.github.com/Zutyosh/4d884ef3d93b582190984d58607e1378
• Proton version: Proton 11 Experimental / Proton 11 beta

Additional comparison:

• Proton-CachyOS-SLR also reproduces the same issue.
• Proton-CachyOS-SLR version tested: cachyos-11.0-20260506-slr

I confirm:

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

I searched for existing Somnium Space / Somnium Space VR reports in ValveSoftware/Proton and did not find a relevant existing compatibility report.

Symptoms

Somnium Space Launcher starts under Proton, but it always shows a false Server Maintenance screen.

This does not appear to be a real server maintenance or firewall/network issue. Host-side network checks can reach Somnium domains, and the same launcher/config works correctly on Windows.

The launcher is a .NET 8 / WindowsDesktop application:

Microsoft.NETCore.App 8.x
Microsoft.WindowsDesktop.App 8.x
Launcher build observed under Steam prefix: 3.5.4_1
Working Windows comparison/export used launcher build: 3.6.7_1

Under Proton, the launcher appears to fail while decrypting/loading its embedded network config before it can build the remote server URI. The app then falls back to the maintenance screen.

Relevant Linux app log from the Steam prefix:

[Application]: Launcher build - 3.5.4_1
[CryptoHelper]: Failed decrypt! | Padding is invalid and cannot be removed.
Failed detect entropy! | Object reference not set to an instance of an object.
Failed validate entropy value as [EncryptedBytes] from [Byte[]]
[CryptoHelper]: Failed decrypt! | Value cannot be null. (Parameter 'value')
[Application]: Remote server status: [not_available] | Value cannot be null. (Parameter 'uriString')

The Proton log shows the launcher loading .NET 8 crypto assemblies and then throwing repeated .NET exceptions with System.Security.Cryptography.dll frames:

Command: ['/mnt/SSD2/Somnium Space/Somnium Space Launcher.exe']
Effective WINEDEBUG: +timestamp,+pid,+tid,+seh,+unwind,+threadname,+debugstr,+loaddll,+mscoree
Loaded ... Microsoft.NETCore.App\8.0.12\System.Security.Cryptography.dll
Loaded ... Microsoft.WindowsDesktop.App\8.0.12\System.Security.Cryptography.ProtectedData.dll
Exception 0xe0434352
backtrace: System.Security.Cryptography.dll
Exception 0xc0000005 (EXCEPTION_ACCESS_VIOLATION)

On Windows, the same launcher/config path succeeds and the launcher proceeds normally:

[Application]: Remote server status: [available]
Game service process is ready
[ConfigLoader]: Launcher config load success[5151]
[LaunchControl]: Client arguments sent to game service[1|1110]
Unity client: Arguments loaded from game service successfully.[3569]

Observed behavior:

Proton 11 Experimental / Proton 11 beta:
  Launcher starts, but fails config decrypt and shows false Server Maintenance.

Proton-CachyOS-SLR / cachyos-11.0-20260506-slr:
  Same crypto/decryption failure.

GE-Proton10-34:
  Launcher does not reach the same stage / fails earlier.

Winetricks/tests tried:

dotnetdesktop8:
  Required/kept installed. Launcher reaches the current failure state.

native crypt32 override:
  Tested and made things worse. It caused DPAPI/RPC errors, so it was removed.

winhttp:
  Tested as an extra experiment, no confirmed fix.

The likely failing area is .NET 8 / System.Security.Cryptography behavior under Wine/Proton, resource/entropy loading, AES/CBC padding, or DPAPI/ProtectedData interaction. The false maintenance page is a downstream symptom of the missing/decryption-failed config.

Reproduction

1. Install Somnium Space Launcher / Somnium Space VR.
2. Add Somnium Space Launcher.exe as a non-Steam game.
3. Select Proton 11 Experimental / Proton 11 beta.
4. Add:

PROTON_LOG=1 %command%

1. Launch:

/path/to/Somnium Space/Somnium Space Launcher.exe

1. The launcher window opens.
2. Instead of the normal launcher/home/login flow, it shows Server Maintenance.
3. The launcher app log shows crypto/config decrypt failure and null uriString.

Logs to attach:

steam-15466454116586225664.log
launcher_output_log.log

Additional context from Windows comparison:

Windows launcher build: 3.6.7_1
Launcher -> Game Service:
  Ping
  LoadConfig
  SetArguments

Game Service -> Client:
  Ping
  GetArguments

Working Windows logs show:

Downloaded launcher config[5151]
Received launcher arguments: 1110
Arguments loaded from game service successfully.[3569]
Launch mode: MODE_NON_XR
Client: 3.2.3

The same encrypted launcher config exists on Linux and Windows:

Somnium Space Launcher.dll.config
setting name="config"
base64 length: 128

The Windows launcher decrypts it. Proton does not.

Relevant launcher embedded resources identified:

SomniumSpace.Launcher.DefaultFiles.client_cert.bytes
SomniumSpace.Launcher.DefaultFiles.main_entropy.bytes
SomniumSpace.Launcher.DefaultFiles.release_service_validation.bytes
SomniumSpace.Launcher.DefaultFiles.service_public_cert.bytes
SomniumSpace.Launcher.DefaultFiles.stage_service_validation.bytes
SomniumSpace.Launcher.DefaultFiles.url_key.bytes