Just a quick update from my side. After try and error for many more hours, i had no idea left what could possibly cause this issue. I tried everything from rollback, to fresh installation of unraid (even bought new flash stick in case my one was faulty) runned a memtest for about 3 and a half days, where each test passed without a single error etc.
After trying almost everything i felt really lost because i had no further ideas what could possibly cause this issue. Then i googled a bit for similar issues but not specific for unraid, more for general computer systems (Googled for the symptoms). At some point i found many interesting articles about "half faulty" PSU's that still power the computer / server but do not deliver a constant / correct voltage anymore. I tested this theory by letting the server (unraid 6.10.3) run in safe mode without any dockers or other services, just plain unraid. The server then stayed up for 3 days without any crash. As soon as i rebooted back to normal mode and started some load intensive docker containers, the server crashed again. Then i borrowed an unused power supply from a friend and temporary installed it in the server... and voila! it was working like it did all the month before. So i bought a new power supply, installed it in the server and since then it is up and running again for 4 days. on Monday and yesterday i was intensively stress testing the server. No more crashes, everything back to normal. As the server was still powering on and booting normal, the psu was actually the last thing i thought about. Anyway, really strange issue that almost brought me to the madhouse / nuthouse, but i could finally solve it.
PS: Sorry for my english it is not my motherlanguage