Notebook internal screen fades on switching occasions like standby and fullscreen

Since about one or two weeks, my Thinkpad T420s using Ubuntu 16.04 quite frequently shows a strange error:

On some display changes, like after standby, but even when switching VLC to fullscreen (no mode change!!!) the internal screen fades slowly to a bright dirty grid pattern.

It looks like the LCD screen is unpowered, while the backlight is still on. Beside the screen, the machine works as intended. Even the backlight brightness can be changed by hotkeys.

Some kind of refresh, like switching virtual terminals, usually restores the display immediately.

I will try to downgrade the driver about a month and see if this remedies the error.