Sorry for the necro-threading, but I just googled this »problem« and this thread was the first link.
Depending on your networkmanager setup you can tell calibre to skip the internet connection check:
Go to Preferences->Tweaks and search for skip_network_check and set it
to True and you will be fine.
copy-pasted from:
https://bugs.launchpad.net/calibre/+bug/1970054
Happy reading!