Uploading 15000 files to Proton Drive stops when away from computer with screen locked – MacOS/Safari fault or Proton fault? [Caffeinate is on so the obvious reason, computer going to sleep is ruled out]

I started uploading large number of files to Proton Drive from the Safari. It runs smooth till I lock the screen. I have Caffeinate (an OS:s own command line app that prevents the computer from going to sleep) on, so the obvious reason, computer going to sleep is ruled out.

I noticed also that running the backup on TimeMachine does the same thing – if the screen is locked, backup fails as "some files can not be accessed".

Is this a new security feature of the OS, or Safari's power management stuff or programming error from Proton?