Live and Neutral reversed: cable colours wrong or the socket tester wrong?
The socket you have there is called a &quot;Schuko&quot; (or CEE 7/3 and 7/4, to be formal).
This socket is entirely unpolarized, by nature.  The socket design makes no differentiation between hot and neutral.  The plug can simply be flipped over, and the two are exchanged.
Part of a genuine CE listing (or the third party tested TUV, BSI etc. ratings) is that every appliance must be designed to be safe with either conductor hot.
So the simple answer is that when you get &quot;Hot-Neutral Reverse&quot; on a tester, it means you plugged the tester in upside-down :)   Really, we could do a whole comedy sketch here, because it's entirely a matter of perspective!
Meanwhile, in Canada... NEMA 6 sockets are polarized even though both working contacts are &quot;hot&quot; at 120V and there's no reason whatsoever to polarize them.
Also, you selected a cheap Chinese mail-order tester, we've seen the unit around here before under a different name.  Like all that stuff, it is sold direct-mail, which bypasses the government safety apparatus that keeps equipment safe.  It lacks TUV certification, and the CE is completely phony.  Using counterfeit safety equipment defeats the purpose.