That is a very odd situation indeed.
I can think of multiple things that can cause it. So the way I do things is a make a check list and I start a process of elimination so that I can narrow down what the problem is and not waste time on other areas that wont help.
So the only reason I suggest to try another monitor and monitor cable is to simply mark them off the list. Same goes for the mouse as well.
While it is unlikly it is one of those 3, it is still possible, so lets just make sure and mark them off.
Then after those are tried I would then do some testing on the installed software and drivers. Now you said you did a clean boot, so if it was done properly then no 3rd party programs should have been running in the back ground, if so we can mark that off the list as well.
So at that point we move to drivers. Test how it works in safe mode, if the 2nd monitor will even come up in safe mode, some videos dont show up in safe mode while others will but just cloned of the main. But still test.
Then remove the display drivers completely, and then reboot, dont reinstall the drivers yet. Let windows try to install its own, and even if no drivers are installed, try the 2nd monitor anyways and see how it goes.
If the problem still happens in safe mode OR when the drivers are removed and no new drivers added then we can mark the video drivers off the list.
Process of elimination can take a while sometimes lol
Shane
