The other morning I was greeted by a mailbox full of messages from failed cronjobs.
The reported error message was:

<28>Nov  7 02:51:02 ntpleapfetch[3253838]: Download from https://www.ietf.org/timezones/data/leap-seconds.list failed after 6 attempts
--2024-11-07 02:51:02--  https://www.ietf.org/timezones/data/leap-seconds.list
Resolving www.ietf.org (www.ietf.org)... 2606:4700::6810:2d63, 2606:4700::6810:2c63, 104.16.45.99, ...
Connecting to www.ietf.org (www.ietf.org)|2606:4700::6810:2d63|:443... connected.
HTTP request sent, awaiting response... 404 Not Found
2024-11-07 02:51:02 ERROR 404: Not Found.

The failing cronjobs were weekly invocations of ntpleapfetch to get the latest list of leap seconds.

After some research I found out that indeed the URL returns a 404 and that there was no newer version of the Debian package available to try.
Also the bugtracker didn't show anyone else dealing with this problem.

Thus I started looking at the source code of ntpsec (which provides the ntpleapsec script).
I found a commit with the promising title of Fix URL used by ntpleapfetch.

This commit corrects the URL used for downloading the leap seconds list in the script.
Later I also found a corresponding message in the ntpsec users mailing list.

For my Debian systems there is no updated package with the new URL available yet.
Thus I used the following one-liner to directly fix the ntpleapfetch script.

sed -i -e 's_^LEAPSRC="https://.*"_LEAPSRC="https://data.iana.org/time-zones/tzdb/leap-seconds.list"_' /usr/sbin/ntpleapfetch