I had the same problem and tried all the above to reset and purge the monitor's cache to no effect, but this worked for me and solved the problem.
You can solve this if your monitor is out of warranty, you are adventurous and not afraid to do a bit of dismantling!
I UNPLUGGED ALL CABLES then removed the stand and its stem and found a rubber grommet with a "void if removed" sticker on top. I removed this and the grommet to find a little screw that locked the monitor bezel in place. I removed this and with a guitar plectrum, I prized the bezel off. Behind the buttons, there is a circuit board containing all the switched. Carefully pull out the cable then remove the board from its holders. Pull these gently away from the board to...