HP Gen10 Microserver does not boot after shutdown.

Hello Community,

I have to following problem regarding my Gen10 Microsever.
After am unpredicted shutdown (circuit breaker in my home) my microsever was shut down and since then refuses to boot. During the post screen the code 92 is shown in die down left corner of the screen and the message to enter boot menu or bios is not shown. What I have done so far.
Remove all HDDs, no change.
Remove the internal USB boot device, no change.
move the Ram to the second socket, no change.
Remove CMOS battery, no change.
reset bios with internal jumper, no change.

But what I have been discovered is that the server properly boots unless no usb device such as an keyboard is connected. what could be the cause of this behavior?

Kind regards
K.