Hello,
Thank you very much for your comment, Benoit.
Last week, a Yamaha tech was checking my CVP. At first, he thought that the problem was a faulty amplification board but, after trying with a new one, the problem persisted. Then, he started to check all the wire connections on the left side and noticed that, suddenly, there was some sound from the left speaker (and from the left headphone) when moving the cover panel from open to closed position or the other way round: the bunch of wire connected to the left speaker seemed to be too tight and, depending on the cover panel position, there was some kind of bad contact that caused the problem. The tech just loosened the wires a little... and problem solved.
Pablo