AMD FirePro - second monitor

[Jakob Miksch <info@jakobmiksch.eu>]

Hello everybody,

I am running Debian "Sid" on a desktop PC with the "AMD ATI FirePro 
W5100" graphics card. When I connect a second monitor, Debian recognizes 
it but the monitor stays black. I can even move windows on it, but I 
cannot see them.

On the same computer is a Windows 10 partition on which the connection 
of both monitors works without problem.

I have installed all drivers like described here 
https://wiki.debian.org/AtiHowTo . I also tried to install the 
proprietary driver from the AMD website. Everything without success.

Do you have any idea what the problem could be?

I am happy about any hint.
Thanks and best wishes,
Jakob