Can you set Beta Saber's launch options to "PROTON_LOG=1 %command%", run the game to show the failure, and then upload the resulting steam-620980.log in your Home directory here?
Actually one of our QA people was able to reproduce this. Should be fixed in the next Beta, thank you for reporting.
This should be fixed in the 3.7-6 Beta.
Can confirm it is fixed in 3.7-6 Beta for me.
All load into 'vr desktop' (i.e large resolution not fitting monitor resolution) with no steamvr bridge interoperation at all, no tracking nor display in hmd - or they simply post "no hmd detected". (including white listed games such as Beat Saber)
Proton 3.7.4 Beta was working fine. Proton 3.7.3 official release still works fine.