OK, found the solution to the USB issue: We used a TP Link PoE splitter to power the Edge Captain 12V DC Port. We as well powered the printer with a different PoE Splitter from Digitus (becouse of 24V) on the same switch. As soon as we used a 12V splitter from Digitus for the Captain we where able to connect to the printer without any problems.
We don’t know why but its working now… We will send the TP-Link splitter back and use the Digitus from now on…