RE:
I went mk4->mk4s->core one and printed like ~100h without problems (parts for coreboxx) and now I got the reboot feature.
I set 11h print and went out to work, came back and got to see core one all dark, cold but beeping with BSOD on the screen (screenshoot included). The BSOD codes of course won't tell you anything. Though maybe it was once in a time failure. However the next print failed on first layer, heard a beeping and saw Core One rebooting and resuming print.
With the power of google I found this thread, and oh boy, a grounding issue. I scrapped the paint under both screws for X and Y motor, got a normal washer (didn't have a star one at hand for M3 screws) and screw it togheter. Checked with multimeter - it was now grounded and it stopped rebooting.
Thank you noumenonsense for finding out what was the problem.
I am disappointed in Prusa, both with the issue (which should come out in testing and could be easlily solved just by adding star washers) and how it was (not) handled. Issue was reported in the middle of march, so we have 5,5 months without Prusa finding out the root problem, and when the users found out it was a grounding problem with motors in june, Prusa also didn't say anything it or made suggestion to install star washers under motor screws. What is more, this looks like a flaw in design which is pretty bad for a 1049/1349 euro machine (not to mention the VFA issue for which Prusa after 6 months says just tighten the belts more).
RE: Core One crash/restart
I am disappointed in Prusa, both with the issue (which should come out in testing and could be easlily solved just by adding star washers) and how it was (not) handled. Issue was reported in the middle of march, so we have 5,5 months without Prusa finding out the root problem, and when the users found out it was a grounding problem with motors in june, Prusa also didn't say anything it or made suggestion to install star washers under motor screws.
It seems Prusa doesn't monitor this forum, at least it appears that way. Wonder if Prusa knows about it or not? I guess one one to inform them is to add an incident to the firmware github repo.
RE:
The fix really is removing two screws, one for each motor, easily accessible without any disassembly, scrape some paint off, add washers to the screws for better long term results, and install the screws to their original locations.
ps: I took off the bays because I wanted to use a flat pilot drill bit (I used a 1/4" / 6mm bit) that would cut the paint and not the metal... the bit needs to be vertical to work as intended. But the scraping could have been done with an Exacto knife.
RE: Core One crash/restart
I really appreciate the time everyone has put into this thread. I've been dealing with the hot-end noise confusing the loadcell issue which at least doesn't spoil a print ("only" prevents printing). That seems minor in comparison, especially since it can (at least for me) be avoided in g-code.
Here in coastal New England we're heading to Fall/Winter where I'll be dealing with much lower humidity than in the summer so I'm off to scratch some paint and put on some star washers. Thank you all!