I have the same problem.
Everything works fine under Windows and WAMP.
I add on my blog
https://pokern24.org/wprss
the rss-Super-Reader, the already mentioned error message appears.
Now I’ve also tried the whole thing in a staging environment. All other plugins switched off and still the same error appears.
SSL is still valid – I checked.
The attempt to solve the whole thing with http instead of https fails for me
