Thanks, I already tried the firewall test, but the product I use (ESET Smart Security Premium) does not prevent programs from opening sockets, it just blocks all of the traffic to/from those sockets.
And, strangely, I did nothing at all -- I left the machine running when I went to work -- and when I got home and rebooted once more, the content server worked fine. Windows was no longer preventing access to the port I use due to permissions.
So, the problem is gone, but IMO without any actual resolution. However, it was surely a Windows issue, not calibre. I only posted here in case it was a common thing after the most recent Windows update. Thanks for taking the time to think about it!
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