U2713H - not longer recognized by Ultrasharp Calibration Software

Hi,

after my system and my display was working fine for a while, one of my "old" problems is back. I don´t know what was the root cause but I am not able to calibrate my display any longer. every time if I start my Dell Calibration Tool, it tells me to connect the the display to the usb port or to check the connection. as far as I know, last time there was an update for the Display Manager available which I did install. What I already tested.

I deinstalled the calibration Software and reinstalled it.
I also reinstalled the display driver. 

Some times ago, I was facing exactly the same Problem but this time I am not able to find a solution. Please help !! OS: Windows 7 64bit