I did not realize, I could simply try safe mode without any privileges.
After that was pointed out to me, I observed that
- a) Reactions are a plugin, so disabling all plugins got rid of the problem in an unhelpful way ^^
- b) More importantly: With no themes, everything works fine.
The only theme that should have any interaction with reactions is
https://github.com/VaperinaDEV/discourse-custom-reactions
The reactions definitely worked correctly a few weeks ago and that theme has not changed in years.
Either it interacts weirdly with a recent discourse update or something broke in our configuration. I will try to inquire more.