@leocg Okay, I think we're getting somewhere. I now opened up the shortcuts setting in Opera GX for the first time, and:

So, for Zoom in, it's not <ctrl>+<+>, but it is one of those. Eg. <Ctrl>+<=>.
BUT...
That doesn't work either.
To get "=", I have to push <shift>+<0> (zero). When I thus push <ctrl>+<shift>+<0>, nothing happpens.
I now have pushed on the "Reset all" button and it didn't change anything at all (at least not as far as "Zoom in" is concerned; haven't checked other shortcuts).
The question still stands - how to zoom in? "+" and "=" don't work.
I now added new shortcuts <Ctrl>+<Shift>+<1> & <Ctrl>+<Shift>+<0> (because <Shift>+<1> yields a <+> and <Shift>+<1> yields <=>). With these, zooming in works.
Is that really the way to go? Having to manually define shortcuts to be able to zoom in?