@Brad Saucier: yes, I managed to make it work again, but I confirm that the connection management is weak.
You should not be obliged to restart piano (and in any case it could not work, as I told)
Also the fact that it confirms connected but than it does not work means that something must be fixed (and the message that I don't have a compatible piano is a non-sense)
Besides, my mobile is new and performing, so it is not questionable (for compatibility or responsivity on the interface).
My invitation for the company to work either on the app or on the piano fw, to make it more mature.
Thanks Brad ;-)