Do not manage to activate HDMI on a laptop (that has optimus / bumblebee)

I am trying to use the HDMI output on a PC (HP ZBook) with Debian (stretch). I have configured Bumblebee, it works well (glxinfo and optirun glxinfo report the expected information, and I tested complicated GLSL shaders that also work as expected). Now I would like to be able to plug a videoprojector on the HDMI. I have read here that intel-virtual-output can be used to configure it when the HDMI is connected on the NVidia board (using a VIRTUAL output that can be manipulated by xrandr). However, intel-virtual-output says: no VIRTUAL outputs on ":0" When I do xrandr -q, there is no VIRTUAL output listed, I only have: Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192 eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 345mm x 194mm 1920x1080 60.02*+ 59.93 1680x1050 59.95 59.88 1600x1024 60.17 ... other video modes ... 400x300 60.32 56.34 320x240 60.05 DP-1 disconnected (normal left inverted right x axis y axis) HDMI-1 disconnected (normal left inverted right x axis y axis) DP-2 disconnected (normal left inverted right x axis y axis) HDMI-2 disconnected (normal left inverted right x axis y axis) My installed version of xserver-xorg-video-intel is: xserver-xorg-video-intel_2.99.917+git20160706-1_amd64.deb **Update (Sat. Dec. 09 2016)** I have updated Debian, and now X crashes when second monitor is active when I starting some applications (for instance xemacs). Sat. Dec. 17 2016: Yes, found out ! (updated the answer). **Update (Wed Sep 27 2017)** The method works in 99% of the cases, but last week I tried a beamer that only accepts 50Hz modes, and could not get anything else than 60Hz (so it did not work). Anybody knows how to force 50Hz modes ? **Update (Tue 01 Oct 2019)** Argh! Broken again: After updating X and the NVidia driver, optirun now crashes (/var/log/Xorg.8.log says crash in Xorg, OsLookupColor+0x139). **Update (07 Oct 2019)** Found a temporary fix (updated answer). https://github.com/Bumblebee-Project/Bumblebee/wiki/Multi-monitor-setup