Are you using an active or a passive adapter? It sounds like a compatibility issue with the adapter used.
Thank you very much for your intention to help, I do not know if the adapter is active or passive.
But meanwhile I had found a simple and good working solution here:
Richtige Antwort von clipped.osprey am 21.07.2015 16:29
I managed to fix it!
Turns out that the AMD drivers only support a pixel clock of up to a certain rate. Monitors that run at this resolution at 60 fps need a higher pixel clock rate.
ToastyX has a patch that fixes this problem. Available at: AMD/ATI Pixel Clock Patcher.