Problem with one of my DSO201

I do have 2 of this nice Sopes on the Workbench.
Both Devices are brand new.
One of them works well with the 1.4 Firmware. But the second one (the same firmware) does not work.
He did not proceed the “green” Signal Line on the screen.
So its impossible to see an Input Signal.
Any Idea what should be the reason?
I need some help.