Hello friends of Opera GX!
I am a user like you who also meet stopped working and after investigating I found the Opera reddit where they give the solution to the meet problem here it is:
@musi11829 Just search user agent in the store page. Try the ones updated more recently.
One that can change the UA on a sites basis may be more interesting.
If you go to the Chrome Web Store (need Opera's Install Chrome Extensions as well) you can then download an app called Bookmarks clean up. It will allow you to delete duplicates, broken urls etc. Far easier than doing it manually.