Hi bll and welcome to the forum.
You should contactsupport as you may require a repair under the guarantee or other guidance.
I would recommend removing the battery. I realise you’ve done that already, but this time wait 15 minutes before re-inserting it. Test the phone and then turn it off again before replacing the back cover if the problem is solved.
This could also be due to a poor electrical contact.
Your case sounds similar to the one below (not sure how that ended but I guess @pfreudenthaler probably did a factory reset).