Headphone jacket issue DJM-900NXS2

Hello !

I found a problem today on my full nxs2 set.
when I plug in my headphones (HDJ-X7), the sound on channel 3 starts to cut. the problem doesn't appear on channel 2. (1 and 4 are free).

And the 3,5'' jack port on the DJM is working good. I found it very strange.
any idea's what causes the problem ?

I've uploaded a ''hidden'' video to youtube for your understandings :

https://youtu.be/ClGhwP53Zyw

Thanks in advance !!