Okay. Rolling back the CSS did nothing. It was a longshot anyway.
I was now also able to confirm that embedding is broken in general. The English side of the blog also stopped working. New pages are stuck at “Loading discussion…”
I am now pretty sure this broke when my provider updated my forum to 3.0.4 but I do not know what version they updated me from. So I still suspect this patch somehow:
https://github.com/discourse/discourse/commit/ccb345bd88024edf79fc91f90f02c5aac897a8d4
I am thinking the fact that the browser is tossing these DOM-related errors is no accident:
Turning off CSP in the admin settings doesn’t seem to fix it.