Q: Video output stopped working on MacBook Pro

Yesterday the two DisplayPort outputs on my Late 2015 MacBook Pro stopped working. There have been no hardware or software changes, to either the laptop or the monitors (Dell U2715). They've been working fine for months, and now nothing. When I plug in either monitor OS X (El Cap v10.11.5) doesn't seem to even notice - no flickering, flashing, etc. The monitors briefly wake up, as if they detect an electrical signal, but then detect no input and go back to sleep.

I've tried a few things...

1. Restarted a few times
2. Restarted and reset NVRAM/PRAM
3. Forced OS X to create a new com.apple.windowserver.[guid].plist file from ~/Library/Preferences/ByHost/.
4. tail-ed a bunch of logs from /var/log while performing various display tasks (unplug, plug in, change built-in LCD settings) and nothing seems to indicate a display problem. In fact there's no logs written at all when physically connecting displays. The ONLY log that's remotely display related is from system.log, and states "DMProxy[64795]: AMBD Services: _CFXPCCreateXPCObjectFromCFObject failed!!". But this error repeats frequently and goes back the oldest log file from last week, when the displays were working just fine.
5. Connected to different external displays with the Mac and also received no output and no system logs.
6. The previously-working setup was all DisplayPort connections, on both ends. I've tried using Apple-branded Thunderbolt->VGA and ->HDMI adapters to connect to different displays, all with no output.
7. I connected a Thunderbolt->Gigabit adapter to the same ports, and it was detected by the OS.

Edit: The HDMI output is also not working, and was working last week. So, apparently all video output has failed...