Thank you for mentioning the
Open With feature, I didn't know that before. I tried to select Acrobat Reader via the suggested readers and via manually browsing as well (it seems to differentiate there, since the logos are different), but neither of them did work.
I reinstalled Acrobat Reader and cleaned the Registry, but that didn't help either (Although I'm not sure if that worked correctly, since it showed the last used files...). Afterwards, I tried selecting it via
Open With again as mentioned above, without success.
Interesting is that it does work when I click
Open With --> Adobe Acrobat Reader DC, but not by doubleclicking the book like I always tried before.
The error message I always recieve is
this (hope this works). It just stays blank, and after a short time, Calibre won't react anymore.
Another thing that might help diagnosing the problem: I recieved once an error message that said something about "module propsys" or similar, but I can't replicate it currently, and it also just appeared for a very short time before closing itself.