You should take it to service center. You still have your phone under warranty.I recently got a k850 secondhand. It starts up with the SE logo then goes into a continuous loop of restarts where the keypad and camera lens ring lights up and will continue to do so until the battery is pulled. I have tried to do an SEUS update and the phone is apparently up to date with the latest. Also I've tried to see if the erom can be repaired by the wotan client but it doesnt support the DB something something.
Look all phones have good sides and bad sides. You know, what is important? That are more good sides.k850 has many mistakes in construction and firmware, i'm a bit disappointed every single day a find a new one.
Nice day