Unit system time 5 years behind

Hello,

I installed the 1.6.15 software and the unit keeps asking to run the update system time routine every time I start it up.
After running the routine the system time is correct (f.i. 6 may 2026), but after shutdown and restarting the unit it falls back to the year 2021.

Is this a software problem other people recognise or is this only my unit and backup battery or something like that? ?

regards Rob