etckeeper (go look it up, it’s very useful) is a tool I use. Now, some behavior in
cron changed recently in Debian and apparently also Ubuntu, causing my log files to show the same as
fetchmail in the bug report.
etckeeper returns an exit status of 1, if nothing has changed. Therefore whenever no changes would show up, I’d see this warning message but could not discern any details from it.
The fix was easy enough and is described in a reply to the bug report.