If you look at Help>About Opera on your Opera 12.18 installation, that should tell you where the Opera 12.18 folders are.
Don't delete any of them! Any other Opera folders will be from Opera 36, and it should be safe to delete them.
If you want to be absolutely safe, don't actually delete them, just move them somewhere so you can restore them later if anything goes wrong.
I just re-discovered that I'd been using a custom style sheet for ebay, which I guess is the source of this no longer working.
It was given me in the old days when there was still an Opera community, so I can't just recall the dialogue as to how I got it/who from/why.
In case anyone stumbles on this thread, I just used the previous style sheet - which also failed (not surprising, as it failed ages ago).
So then tried NO style sheet - and success!
The only nagging doubt is trying to remember why I needed a special one in the first place...