Dell Latitude 7370 with Ubuntu 16.04 won't display on second monitor

Dell Latitude 7370 with Ubuntu 16.04 won't display on second monitor. My desktop monitor that I'm trying to connect to is an LG 34UB67. The LG recognizes that the laptop is connected, e.g. if I switch the HDMI cable from port 1 to port 2, the LG monitor asks if I want to automatically switch to the input from port 2. It just doesn't display anything and after a while says there's no signal. My colleague, who has the exact same laptop, also struggles connecting to projectors etc. This isn't very concrete and possibly not very helpful, but he says that the second monitor display worked well until about a month ago, so perhaps a driver package upgrade derailed things.

Graphics: Intel SkyLake with i915_bpo driver. My Dell Inspiron 5347 connects to the LG monitor without any issue (Hawell-ULT integrated graphics with i915 driver). Anyone have any advice?