Hello,
problem solved :-)
I recently installed the Software package "Mission Planner" with is a tool related to UAVCAN. This tool also installs a NXP USB-Serial driver (maybe to be compatible to the NXP drone kit which uses UAVCAN...)
The drone kit form NXP uses a different microcontroller than I use, but the NXP serial driver accepts the LCP54618 in USB MSD bootloader mode as a compatible device for the serial-usb connection.
Uninstalling and removing the driver from the PC solves the problem.