You are the hero! You found it! 
In hindsight, it’s clear again which part of the puzzle was messing it up. But scripting and script-execution sequences are slightly out of my comfort-zone, so I didn’t see it.
Thank you, huge amounts! 
For future reference or others that might run into this bit of weird behavior: if you also use Iubenda for cookie blocking you can add:
data-cmp-ab="2"
to the script tag. This tells Iubenda that this script is harmless and doesn’t block it (and hence doesn’t reload it again, after accepting the cookies).