“Do you think that the case could be the culprit / change things here?”
OK, I’m afraid it’s gonna be a bit difficult to me to explain it in English:
I noticed that while I was pushing the jack connector in very slowly, at a very short moment both channels seemed to work simultaneously (but with extreme distorsion). but only one left when finally full in.
That’s the reason why indeed I thought the case may be involved.
Hope that my explanation is comprehensible enough though.