I am also having this problem, it seems to occur right after Vivaldi finished updating.
This is the third time it has happened, so I am hoping to find some help here to identify the issue:
The first time it happened to me it was just the BitWarden extension, but today it also broke the DarkReader and SteamDB extensions, so it seems to be a general problem with how the extensions are managed.
Current Vivaldi version is 7.5.3735.44
I checked the folder access as mentioned earlier, it seems the access is all correctly configured.
Disabling & re-enabling the extension does not resolve the problem, I have to completely remove and re-install the extension to get it to work.
But it is annoying to keep having to do that every time they break, since all settings for the extension are then lost (this is the third time this problem occured to me).
I re-installed BitWarden since I need that for doing anything, but I left the other 2 extensions untouched, is there something I could try to identify the issue?
I would like to help find the root problem so it does not occur again later.