Originally Posted by kathy jh
It was the eset firewall. In interactive mode I managed to set rules for incoming and outgoing communication between calibre and CC. The connection now also works with the firewall active. Same holds for the ASUS sync feature that wasn't working because of the firewall but that's beside the topic
Thank you for your support.
Thanks for reporting back. I am glad you got it working.
I have added a mention of ESET to CC's connection FAQ
in hopes that others can profit from your experience.