Can't add new broadcast messages


When I try to add a new broadcast message (in admin mode), I get a weird behaviour. I can enter the message, select type and colour for it etc. but when I try to enter a start/stop time I can enter an hour and then focus is moved to the minute field, but as soon as I start to type something the hour field is changed to “–” (the same as it contains before I enter anything, judging from what I’ve seen at my fellow administrators, this is actually where the weirdness begins).

I use firefox and thought it might be one of my extensions (my suspicions were primarily about blocking foreign javascript), but I see the same behaviour in a clean firefox running from a flatpak.

What can cause that and how do I get normal behaviour?