The Windows Mixed Reality Portal on my Dell 5577 laptop quit working some months ago: "unable to start, error code 14-2". That is a catch all code for low level system device errors. Recommended fix tends to be "unplug USB hubs or load latest video drivers". Which of course I did to no avail. This condition persisted through several Win10 updates.
Steam VR (which is the actual driver for the WMR hmd) kept saying, "due to a recent crash, SteamVR is running in Safe Mode" and would not go back to normal mode. Although it did seem to recognize the headset OK, there was no picture, so naturally I suspected the hmd had gone bad. Samsung, to their great credit, sent me a free new Odyssey to replace it. That made Steam VR happy, and the Odyssey was able to accept a firmware update, but WMR still failed.
The culprit turns out to have been a useless piece of bloatware called True Color, formerly distributed by Dell and promoted as part of their s/w maintenance program. Uninstalling it made WMR Portal start OK.