For what it's worth, I have similar boot issues on one of my IBM/Lenovo x3100M4 server. If some types of USB devices are connected during boot, the system will reboot about 1 second after the the grub menu disappears. USB keyboard and mouse boot fine. USB audio DAC, webcam and ethernet dongle always reboot. USB hub boots fine as long as it's un-powered. If it's powered then the system hangs after the Grub prompt disappears (no reboot). All of these devices are fine if I plug them in once the kernel is up and running.

Not sure if it's a grub problem or a kernel problem...the lack of any kernel messages on the serial port suggest that perhaps the kernel hasn't started booting yet.