Windows 7 guest on ubuntu host - second monitor stopped work

[RudolfVonKrugstein]

Hey,

I have installed a windows 7 guest on a ubuntu host. Everything worked, also with 2 monitors.

But suddenly the second monitor stopped working. It shows only black.
It seems the contents are simply not displayed, as the mouse courses changes on certain areas of the monitor and I can drag windows to and from it.

It worked before, what could have changed?
I do not know if it is related: I played around with 2d and 3d acceleration. But I have set all settings back to the original ones ...

Thanks!
Nathan