BTW, I apologize if my question is a repetition of older posts. Especially, due to the FAQ answer.
In my defense, I'd like to say that the one thing I cannot change on my pc is the antivirus service (admin settings prevent me from disabling it). I closed/disabled nearly everything else and it didn't help. On the other hand, I could start the content server in safe mode with networking. It could be concluded that the AV was blocking Calibre, but not with 100% certainty.
Maybe, the server startup routine should give out more info on errors during startup?