I have never been able to use Boxcryptor with Quicken. I was running MacOS Catalina, and am now running Big Sur, same issue:
Quicken Version 6.0.1 (Build 600.37045.100)
MacOS 11.0.1 (20B29) - Big Sur
Boxcryptor Version 2.37.1043
It thinks it has a permission issue, but I have full permissions on the file, directory, etc. If I move the file to my local drive, no problems, opens fine, then I move it back to my Boxcryptor drive and I can't open it. It's not a corrupt file. I'm not sure if this is a Boxcryptor issue or a Quicken issue, but figured I would start here. Anyone have any ideas?