No HDMI signal after Win 10 upgrade

Hi,

My Dell XPS 15 9530 from 2014 no longer seems to be able to connect via HDMI to TV or projector since I upgraded from Win 8 to Win 10. It worked before. I get "no signal" on the second screens.

Spent half a day already on this, googling and trying the following:

* Updating to the latest Dell drivers, Windows updates, and Nvidia updates.
* Disabling either of Intel (HD Graphics 4600) or Nvidia (GT 750M) graphics cards
* Rolling back to older versions of display drivers
* Trying different HDMI cables
* Running Dell's diagnostics tool
* Lowering screen resolution
* Restarting everything

But nothing works.

The external screens don't show up in Screen Properties. Pressing Fn+F8 brings up the four choises (Duplicate etc.) but nothing happens when I click any of those options.

Some posts I found via googling seem to suggest that Win 10 simply isn't compatible with HDMI in some laptop/graphic card/CPU configurations. Can this really be the case, or what can I do?