Mine private Jolla did just the same thing last evening. I had the browser open and suddenly screen went black and it died. I pressed the power button and it started again. When the phone was ready I opened the browser from where I was before and after about three seconds the screen went again black, led lighted up as red and it started reboot loop. I let it run in boot loop for few minutes and then something changed, it started up normally. When it was up again, I decided to shut it down and let the battery have some fresh air. After about two minutes I reassembled the phone and started it.

Has been working flawlessly since then. Did not seem to be too serious. Maybe I should have let it run when it finally exited the boot loop. Might have worked. Well, maybe next time.

I have two Jollas in more or less active use and this is the first time one has been acting up.