I bought a USB-C to displayport cable and plugged into my 2017 rMBP, it worked for a while, then stopped working. If you open the "Display" system preference, there is the external monitor, but the display is blank. Actually, the monitor shows something connected, then went to sleep. Similar problems were reported https://apple.stackexchange.com/que...nal-monitor-black-screen-usb-c-to-displayport, but nobody has a working solution. In my case, reset the SMC, and then reboot with display connected, sometimes it works. Suspecting this is more like a software problem and more related to displayport, since usb-c to HDMI works reliably, and I saw this on displaylink.com.