update your video card driver - unplug monitor if possible
Ah, my friend the video driver. Updated it to the most recent - Single Red Diagonal Line didn't go away. But Photoshop stopped opening images! Now THAT'S a "feature".
Rolled back to the original driver. Photoshop now works again.
*****But get this.******
Noticed some layers had effects on them that I decided not to use and had turned OFF, but left in the effects stack at the very top.
Picked a layer. Turned back ON the effect (Fill) and the Single Red Diagonal Line went away. (Comp now looked wrong because it had a Fill in it, but that was to be expected.)
Turned the FIll back OFF... and the Single Red Diagonal Line came BACK.
Moved the Fill (turned off) to the LAST position in the effects stack as opposed to from the Top. ... and the Single Red Diagonal Line went away.
Deleted that effect (Fill) and voila... the Single Red Diagonal Line went away.
But other layers also with "off" unused effects DON'T reprdoduce this error.
In the end I deleted the effect and all was good with the Universe again.