SOLVED
Hi all,
and thank you very much for your replies!
Indeed the skin myMetrixHD v1.5 (which I used since years without any problem) has caused the issue. Since I switched back to the openPLI standard skin and de-installed the myMetrixHD v1.5 skin, both boxes are running absolutely stable again - in normal operation mode as well as in normal standby.
Thank you very much again and best regards
Jens