** Solved **
For anyone else facing the same dilemma killing the opera process tree or alternatively restarting your PC after updating to the latest version of Opera should fix the problem. It did so for me.
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Hello, World! ...
I've recently migrated to opera, as an active alternative to Vivaldi, which is an active alternative to chrome..yada yada yada...
My problem I'm having is I'm unable to add any extensions or themes etc..
I can browse the catalogs fine but when I press the "Add to Opera" button for the chosen extension/theme nothing happens...
I tried updating to the latest version, but that didn't fix the problem.
For the record I'm on Windows 10 (Enterprise)
Thanks for any help!