Honestly, if you were a geek I'd suggest uninstalling and reinstalling the dictionary files, but instead, I'll suggest trying to save or export any bookmarks/favorites and do a complete uninstall of the software and reinstall.
I don't personally use Opera nor do I use Windows so I can't give more specific advice than that.
...a complete uninstall of the software and reinstall.
***
That could be a disaster, so I’ll stay away from that option.
I have now disabled spelling, so I’ll just have to be much more careful with my typing and my proofreading, I suppose.
Thank you.