I just did some testing and some research. Same solution as the same error on Google Chrome.

The problem comes back after a while so developers please do something.

Not sure if its temporary or permanent solution but it worked for me

Steps:

1. Close all Opera GX related processes.
2. Go to C:\Users\username\AppData\Roaming\Opera Software\Opera GX Stable.
3. Copy and Paste "Webdata" and "Webdata-journal" somewhere and keep it safe.
4. Delete "Webdata" and "Webdata-journal".
5. See for yourself.
6. If it doesn't work, return "Webdata" and "Webdata-journal".

Side effects:

• My search engine returned to Google

• Extensions removed