thanks a lot for your answer
I sent the exact same message to the french support the same day I posted on this forum
They answered, advising me to check the USB port on my computer as well as on the device... and the cable.
The app checks the validity of the license at each usb connection. So if there are micro "cuts" in the connection, It can end up jamming the all thing.
And I must say that I find the USB port on the interface paticularly loose... as in "not tight"
I will change the cable to see if it does the tric by "gripping" its socket more efficiently...
I buy one tomorrow and let you know if the glitch comes back !
Thanks for your help.