BTW in the end it was likely a water damage (“oxidation”) and the support sent me this, TBH quite professional, reply:
The first link there points to a very good explanation/article of iFixIt that also debunks the myth of the rice against water:
Thankfully, the FP4 is some more water-resistant, but to be sure I’ll likely still stay cautious now.
Anyway, really a bad bad thing.