[10.11.1] [Macbook Pro mid14] Thinkvision monitor won't work now.

Macbook Pro Retina mid 2014, El Capitan 10.11.1, Lenovo L194W monitor

It may be coincidence, but after updating to El Capitan, the external monitor connected via HDMI>DVI cable stopped working.

So we tried it via various methods and here's what happens - "problem" means that when a display is connected, the Mac display flicks off for a second and back on.

HDMI > DVI cable on Lenovo: problem

Alternative HDMI > DVI cable on Lenovo: problem

Thunderbolt > DVI cable on Lenovo: problem

HDMI > HDMI on a TV: OK

Thunderbolt > HDMI on an Acer monitor: OK

HDMI > DVI on an Acer monitor: Display flashes 5 times, Macbook crashes.

A PC via HDMI > DVI on the Lenovo monitor: OK

An older Macbook via Displayport on the Lenovo monitor: OK

Crazy, huh? So there's nothing wrong with the Lenovo monitor, there's nothing wrong with the Macbook Pro on an Acer monitor, but Macbook Pro and Lenovo monitor - Lenovo-no-go!

Have tried PRAM and SMC resets, too.

Any ideas?