Update from my end. The Corsair memory won't even let the machine boot. My last attempt was than to order also Samsung memory. After almost 6 hours (4 passes) of memtest it didn't found any errors. So now my N100 is running on OpnSense.
I assume (and hope) it will be rock solid now. Thank you all for your hints and tips. :)
I assume (and hope) it will be rock solid now. Thank you all for your hints and tips. :)