In the “Displays” panel of System Preferences I clicked on “Gather Windows” so I could access the settings for the Philips. There I clicked on “Default for display” and all is fine now. Obviously the Philips 17S doesn’t work with the 75 Hertz that were set by default.
After working through the framebuffer patching, which was all new to me, I was in the “think complicated mode” as it seems. That wrong settings in the System Preferences could lead to the screen staying black simply didn’t come to my mind – until it suddenly did now.
Wow, I’m glad this works now!
By the way, I have managed to create the EDID patches. For the main monitor (Dell) I used
this script (which only does it for the first screen) and for the Philips I did it manually according to
this guide. Only strange thing is that the Dell isn’t recognised by the system at all if connected via DVI, while the Philips also works via VGA, but only with the glitches described earlier (even after EDID patching).
Anyhow, I again want to thank you for your patience and for your effort in sharing your knowledge here!
EDIT:
So for VGA and DVI I can confirm that Peter’s framebuffer patch for the Gigabyte H370M D3H is correct (though not for every screen, for whatever reason). But you might want to mention that it is also applicable to the Gigabyte H370M D3H
GSM and you might want to add a variant with a different platform ID as I use it.
See
post 460 for the exact settings. IIRC, I also tried it with “HDMI20 (4K)” and even “VRAM 2048 MB” checked and it didn’t make a difference.
EDIT 2:
And you might also want to mention in the guide or at the patch for this motherboard that this solution works on High Sierra too.