Possible grounding issue on a '05 JP6 (with Piezo)

Hi everyone!
First of all, Just got my first JP6, a guitar i’ve been dreaming for almost 20 years, and decided to register to the EB forums.
The guitar I bought is made in 2005 and has Piezo system.

For the first couple of weeks the guitar worked great! Felt and sounded great.
But after that the Stereo/Magnetic input started a loud hum everytime I release my hands from strings or metal parts.
Also, the volume and tone almost completely disappeared when using the Stereo input.

I just opened the back cavities and got really really confuced, haha.
I’m newbie on this matter and have very little experience with guitar electronics.
Can anyone help on where I should begin to try to locate the problem?

Thanks! 🙂