Good People:
Calibre and Kindle ( or other brands) are not the only players in the 'get the book to my device game"
So the "I haven't changed " anything is or isn't relevant depending on your particular system / network configuration.
GMX changed their Terms of Service (recent coverage in other threads) and you need to accept (outside of Calibre) for you to be able to continue usage.
Microsoft sent (and I installed) a 225mb to my W7 laptop system yesterday
W10 took the choice away. It happens.
Your security suite gets daily (or more frequent) updates, usually automatic
The firewall may have changed blocking rules.
Your Kindle can phone home for sync and it might grab the occasional (yearly) update. (The Kindle app on Android seems to update weekly)
Your ISP
may manage their (your end) routers firewall and make silent updates.
All I am saying here is:
re check all the pieces suggested in the Calibre FAQ on connectivity
read the recent Calibre threads here at MR. to see if it has been reported (and a Solution posted)
Check device connectivity BEFORE updating Calibre (or the Device) so you get warm-fuzzies
of what it ISN'T
Sorry for the rant, but we answer the same questions many times and it gets tiresome and takes away from those that do have issues that have not been covered
And
if you do find an issue that is not reported (usually by power users) during the first
release day, do POST the error log/message (in [spoiler] tags)