I could identify the problem! Its not a faulty mainboard, its a cheap and buggy ethernet controller which asus is putting on their (B550) boards (Intel I225-V v2). There is an incompatibility between my Asus EA-AC87 media-bridge and the mainboard ethernet adapter. The funny thing is, the problem only occurs when the new driver is installed and the cpu is on high load. I absolutely cannot understand this kind of issue but yeah, it completly dissapears when iam using another switch/router connected to my board or reinstalling the old driver. Very strange and annoying! Now iam allowed to buy a new media-bridge/switch too, really great... not. Thank you Asus (again) and Intel! I am curious what the support will say about this!?!