Multiple Monitors? Mac Pro 2010 Radeon RX 580

My Mac Pro (2010 / 5.1) was running fine under OS X 10.13 with 3 Monitors connected to it (using the display-ports & dvi of my Sapphire RX 580 - 8 G).

The RX 580 is recommended by Apple as Mojave compatible, but...

After updating to Mojave it´s impossible to bring 3 Monitors alive.

Monitors connected to the displayports will be shown as connected (can see them in system settings "monitors", when i use "collect windows", can move the mouse into) but they stay black.

Even their resolution is far below the specs of graphic cards max. resolution.

No way to bring them alive.

Tried all I know (PRAM/VRAM Reset, different cables, etc...)

Any experience?