Firmware Ver. 4.03 (DV91000107T: released on 26 Apr., 2021) has the following problem.
The camera does not work after updating from Ver. 2.53-* or earlier to Ver. 4.03.
(* Ver. 2.53- DV91000107P: released in Oct., 2018) There is no problem in updating from Ver.3.
This problem has been fixed in Ver.4.04. ｡｡