Gray center, dark sides - warms up to normal after 10 minutes

Hello!

When I first turn the monitor after several hours off, the screen is whitish. It's possible to read the text, but black really looks like gray.

After a while, it starts to fix itself starting from the sides, and after 5-10 minutes turns completely normal, with deep backs and nearly perfect colors.

Several seconds after power up

25 minutes after power up (normal)

Thank a lot for any ideas how this can be fixed!