Phone [Q] HTC One (M7)- screen turns dark automaticly

Hello,

i got a HTC One (m7) and i've installed the latest update,

now my screen automaticly turns dark (no brightness at all)
first thing i thougt it was a problem with the savemode so i turned that of but it stayed the same.
it doesn't mather if i switch the brightnes between 0% and 100% it stayes dark

so i thought my backlight must be broken but....
when i restart my phone th welcome/popup screen is bright and as soon as it's started up again its back to dark.
when i start whatsup the screenn is dark but if i say i want to send a video the videoscreen starts and the screen lights up fully,
when i then go back into whatsup or home you see the sreen go back to dark gradualy in about 3 seconds.

what the hack can do this and how do i kill this so i can get control again
i hope anyone has an idea for me.