dual quadro p4000 with 7x4k portrait as video wall - mosaic not working, black screen

Hi There,

We have a new Windows 10 machine with 2 Quattro P4000 Cards linked via SLI cable.

In the Nvidia Settings both cards and all 7 connected 4k Screens show up correctly.
By Default all 7 screens show an extended Windows Desktop.

In the mosaic settings i select all 7 screens, 7x1 configuration, portrait mode.

As soon as i am trying to enable mosaic in the wizard, the screens flicker some times, then the first output of card 1 and 2 show a black screen and all other screens go to no signal.

Well thats it, it stays like this. 2 Black screens, 5 others showing no signal. Even rebooting the machine does not change anything. I see the Windows boot logo and then its black again. I restored a previous windows backup and tried enabling mosaic again, but every time i get the same result.

Any hints what could be the problem?