There's a post about this at https://www.reddit.com/r/browsers/comments/1i310jj/comment/mc00dle/ that says disabling Nord's Antimalware function, Surf protection and Ad + Tracker blocker avoids the issue. Not sure if disabling all of them are needed or not.
With that said, check Nord's settings for those parts or Nord's settings in general to see if there's some exception list you can add to Opera. Also, it'd probably be good to post in the Nord community to see if they can reproduce the issue and see what Nord is doing wrong.