Yesterday, I changed the audio cable and restored the early FW and the 1.0 presets : all seemed to worked fine.
The synth entered in sleep mode within 5 minutes, leaving a sustained note.
Screen went off.
I could switch off and on again and all was still fine.
I then decided to updarde to latest FW version with 1.1 preset after : same behavior : so far so good.
I sustained my note again and the synth wen in sleep mode again, screen off.
I could switch off and on : I still don't know what really happened.
Honestly I prefer to keep a working synth at home rather than sending it back because its doesn't work.
I will do a bunch of test today again : I hope it will be OK.