Thank you very much for your help, I have fixed the issue.
You were wright, there was a midi loop, not from MOTU Express, but from a “thru” option in the Motif Rack XS editor.
In some circonstances, the USB was overloaded by Motif USB midi inputs/outputs and produced issues over others audio and midi devices. It can explained why the problem occured in so different configs.
Finding the loop finaly drives me to the real problem. From the Motif manual : the only working Midi input port for instrument is “port 1”
Page 18
"• Port 1
The tone generator block in the MOTIF-RACK XS can recognize and
use only this port.
When playing the MOTIF-RACK XS sounds from an external MIDI
instrument or computer, you should set the MIDI Port to 1 on the
connected MIDI device or computer"
Modifying the “Motif editor” keyboard input, and disabling “thru”, has fixed all issues 
MOTIF Rack XS is realy an impressive expander but it’s also a complete nightmare to use without tools like Gig Performer !
Realy happy to find a solution to continue to use this powerful device.
Best regards,
José