The TLS certificate for https://gpg4win.de has expired on Friday, August 27, 2021 at 10:02:38 PM (Berlin timzone).

Workaround: go to https://www.gpg4win.org/index-de.html
which has the same contents, but uses the different domain name and thus a different certificate.

We (at Intevation) are on it. (And we also see why we did not notice).

Thanks to the two people who gave us a hint about the problem.