Hi Peter,
Thank you for your prompt reply.
It is a Rock, but made by myself. Asus P10m-ws mainboard with Xeon . Jcat Net Card XE (best sound ever) Jcat USB Card. A WD 2TB hard drive is connected to the USB card. I have already had this checked. I cleaned also the USB cable on the plugs with alcohol.
Switch is a Sotm. The server and the U1Mini are connected to it. Also a Ubiquiti EdgeRouter X SFP for DHCP. SFP goes to the internet.
If the problem occurs I can still access the browser page from the Rock Server and I can still access the U1 with the Lumin app. Everything is still there and accessible on the Roon app Ipad. No error message appears. Only the title timeline fluctuates quickly back and forth (like Night Rider Car 
)