I have tried to connect another monitor with a direct DVI cable on the second DVI port and it works OK, even with two monitors connected (one via HDMi one via DVI). So the problem comes from the DVI/VGA adapter, which make no sense because it has always worked OK on Yosemite and El...
I have installed High Sierra on my PC with the next configuration:
- Gigabyte GA H97M D3H
- Intel Core i5 4690
- Nvidia GTX 970 4 GB
- 8 GB RAM
System works fine, even graphics and sleep... but only with one monitor connected throw HDMI. If I try to connect another monitor with a DVI link...
I have the same problem as you. Connecting the monitor by VGA it doesn't work, so I think it can be the problem. I will try with DVI adapter to VGA (or a HDMI monitor) so, do you think it will works (by an adapter)?