My only guess at the moment is that you took the backup of the documents folder, removed it, uninstalled the game, copy-pasted your backup into documents - And now your game doesn't have writing permissions to that directory, because the new install wasn't the process that created it.
I'd suggest moving /Documents/Larian Studios/ to your desktop and then exit Steam (if that's your platform) and run Steam as admin, then run BG3 to let it create the new folders under documents, and just move your PlayerProfiles folder back in, and graphicsettings.lsx if you want to maintain your old setting configs.
