DN-X 1100 | master fades away automaticly

Hi y’all:)

I bought a used Denon DN-X 1100 to match my DNS 3700’s; the mixer looks as good as new… There’s only one problem…

When I open the fader the volume raise, BUT after a few seconds the MASTER automaticly fades the music down!!! The inputsignal of the DNS 3700 however stays normal and stable…

When I only touch the fader again the music and the levelmeter of the master jumps back to normal loudness, but then the fading down starts allover again!!! This issue appears on all the lines; Line 1, 2, 3 and Line 4…

For an example see the embedded clip…

Anyone?!