Fascinating. I dug in the code and think I found it. The line of code to blame is https://phabricator.wikimedia.org/source/mediawiki/browse/master/includes/specialpage/ChangesListSpecialPage.php;062d935971a96b788953448fc2b150056a41924e$1325. On this server, $wgRCMaxAge is set to 1000, which limits the days you can ask for to values between 0 and 1000 seconds. The default is 1 day, but this is truncated to 1000 seconds. This is added as a new value to the list of possible values.
It would be possible to add a min( 1, … ) to the line of code I pointed to, and to remove values that are larger from the list. But to be honest I don't think it's worth it fiddling with this code.
Apparently I wasn’t subscribed to this task, so I just had a huge revelation when looking at the LocalSettings.php (“no, wait, is that it? really?”) and then I saw that @thiemowmde already came to the same conclusion a week ago :D
I’ve temporarily commented the line, and now RecentChanges and Watchlist look normal. But that doesn’t solve the problem for all the other containers we created with this configuration.