Dim but active screen regardless of brightness setting on SP5

[desperate_housecoder]

My Surface Pro 5 suddenly appeared to have a very dim screen. only when using a flashlight its possible to barely read things and notice the screen is actually still working. Changing the brightness settings has no impact at all.
When using a second monitor via thunderbolt-port - hdmi i can fully use the tablet.
First guess was the display itself so i carefully exchanged it but problem is still there.

Any help is much appreciated!