Intel Motherboards : H170m Pro4 no 3rd monitor after Kaby Lake upgrade

the third monitor (24" ) is connected vie DVI. 

I first had a Skylake i3 and all worked well with 3 monitors. then i upgraded to a Kaby Lake i7 7700K. Everything works. Except the 3rd monitor doesn't display anything. The Intel Driver recognizes it and i can "move"  windows on the black screen. 

Before I installed the Kaby Lake I upgraded to latest BIOS (7.1) and updated all drivers. With the Skylake i3 that worked. 

I tested that 24" monitor on a different PC and it works fine with DVI. I checked all cables, selected input etc. 

I have the suspicion this is a driver or BIOS issue. Or some issue with my new CPU not supporting 3 monitors? 

I re-installed graphics driver to no avail. I checked all BIOS settings, but non seemed to block the DVI. 

Kaby Lake I listed for this MB. 

I also lost the ability to listen to sound via the Monitor speakers (vie DMI). So there seems to be some driver issue. Before that I could listen to monitor speakers. 

any ideas what to test or change? 

I tried to completely de-install the graphics driver, but it is persistent even after re-start. 

When upgrading to Kaby Lake i literally only replaced the CPU, no other changes. there don't seem to be any other problems. 

I thought before i bug the ASRock support I see if the community has some ideas. This is my second ASRock board and all flawless so far. But I have to say Sound through monitor speakers was iffy for the W10 anniversary update. but the anniversary update in august fixed that.