Sound problem from MASTER of DDJ-440 with panning

I have a DDJ-400 since one month. Everything was working fine until yesterday when I noticed that now part of the sound from the MASTER output is lost if there is panning in the audio track.
In the PHONES output of the controller instead I can still hear all the sound, both left and right, correctly.
I checked that the RCA cable and the speaker connected to MASTER are still working properly so those are not the source of the problem.
Any help would be much appreciated.