Laserjet 300 pro M375 - Central Touch Buttons won't work

Hi,

I have a HP LaserJet Pro 300 M375wn with a non working touch pad. When I try to press any central icons nothing happens. If I press the lateral '?', 'cancel', 'return' or 'home' icons they are working fine. Tried the combination right arrow with cancel button , turned on the printer but keeps the same problem (and no sucessfull reset). Any hint on this problem ?

Best regards