Strange USB behaviour; Grub fails to read disks when no USB keyboard is connected

[Matschek]

Hi Grub Folks,

I am using Grub (2.02) to boot a Debian (OpenMediaVault) installed on an USB
stick.
This is working fine as long as a USB keyboard is connected. Sound stange,
but please read on. As soon as I try to start the machine with no other USB
devices (only the USB key) connected Grub fails after reading the config
file and before starting the OS loader:

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Error: failure reading sector 0x802 from 'hd0'.
Error: no such partition.
Error: no such partition.
Error: no such partition.
Error: no such partition.
Error: no such partition.
Press any key to continue...
  Failed to boot both default and fallback entries.
Press any key to continue...
-----


I edited the boot options via "e" and tried several variants but with no
luck. Grub is not able to find its own resources, even an "ls" command
fails, like all internal commands shown with [TAB]:

-----
grub> ls
no such partition.
-----


If I reboot with connected USB keyboard, everything is fine again. Reboot
with a PS/2 keyboard - boot fails again.
If seems grub loses the USB device access somewhere in stage2 (as far as I
understand the config file is read in stage2, which is shown correctly)

What can I do? Is there some debugging/verbose option I can set so see
what's going wrong? Any extra option I should use for grub-install in my
scenario?

Regards,
Matthias Philipp