Thanks a lot for the suggestion. The USB dump is attached.
66 is the last packet before connecting with USB.
142 is the last packet before the fastboot command.
I guess the bootloader has problems with USB autosuspend. When I played around, and executed the fastboot command immediately, it worked. If I kept executing it again and again, it was all ok, but if I waited for a longer while between the fastboot commands, the bootloader would enter the "Press any key to shutdown" state.
I disabled USB autosuspend with
echo -1 > /sys/module/usbcore/parameters/autosuspend
and I