Flashing an Atmega8u2 on a Mightyboard

Hi,
as the Title suggests I've got a problem with my USB-Interface on an Mightyboard Clone which is used as an Controll Board for 3D-Printers. The Interface between the Main Controller,Atmega1260, and the Computer is established with an Atmega8u2. The problem I have is the apperance in the Device Manager. It says: "unknown Device" with a yellow triangle and a Message Pop's up saying that the USB Device is Malfunctioning. Therefore I can't connect to the board.
After some "Googling" I've got the Information that the Controller maybe has "forgotten" it's Firmware for whatever reason. So I've connected it via an Arduino as ISP to the Computer to read in first place the fuses out of the Controller and voila, it works fine. The fuses have the states they should have (E:F4, H:D9, L:FF). I'm also able to flash the thing with a hex file. (Yes I've tried several firmwares from there: "https://github.com/arduino/Arduino/tree/master/hardware/arduino/avr/firmwares/atmegaxxu2" but without any changing). I'm not sure what firmware was there before but I think it's similar or equal to the in the Arduino's and I don't think that the Chip is broken cause I'm able to read and write to it.

Therefore my Question. Does anybody else had this issue and know's a way to solve it?
For those who are interested I attach the link to the Schematic of the Board. The 8u2 is located in the upper left corner:

I sometimes have the same problem when using cheap or long (>5m) USB cables. Have you tried with a different cable?

Have you tried DFU programming it over USB? Does that work?

Pieter

Yes. I've tried it with various Cables and different Computers. Always the same Issue.
I was not able to get into DFU with the mentioned steps.