Are you using librewolf to upload files from your NAS to Proton Drive?
I readily admit I am still not super proficient with flatseal. I spent a lot of time trying to fix this by adjusting the file permissions, but I’m now wondering if it was some other local network setting I missed.
I also don’t use fstab to mount my NAS. I just sign in using Files which creates a smb link. On Firefox/proton drive website I can see the files but I cannot upload them directly to Proton Drive from my NAS using Firefox (or Zen) on bluefin.
I added this edit above. Pasting here in case you are curious. Cheers.