How to troubleshoot during and post Debian 8.5 system failures with external display?

I have had now three incidences of system failures in two weeks when the external display has been connected to it; it does not happen other times - both displays go mad - big display goes gray bright, while the laptop display goes dark - changing TTY not possible, no response taken; only way to power off the laptop at the situation. I think it may be related to my Modesetting graphics, please, see the link below about Intel graphics for the setup. Troubleshoot strategies 1. Take a ssh connection via RJ45 cable next time to the failed system. How can you troubleshoot during the system failure? How can you correct it? What is the most likely cause of the problem? 2. How can you troubleshoot postconditionally? OS: Debian 8.5 Linux kernel: 4.6 backports Hardware: Asus Zenbook UX303UA Intel graphics: here External display: BENQ XL2411 with HDMI cable