i tested all monitors individually
my other two had no problems whatsoever - even after trying all three again
When I tested my main monitor, it went out several times and I had white blocks(?) that sometimes lit up over the whole monitor, but not again the problems that were in the picture
now they are all plugged in again and currently I have no problems - but i think if i restart again like in a few hours or tomorrow it would be the same (had the same thing yesterday)
Unfortunately I cannot test via the CPU because I am using an R7 2700X