Thanks for the suggestion, and I am sorry to tell you it didn't work. I tried keeping F10 pressed while switcing on, tapping it directly after switching on, anything, but it doesn't enter setup.
I also tried resetting CMOS setting by means of the mainboard jumpers (as described in the maintenance manual for this system), but no results.
I also found a ROM recovery jumper on the mainboard but cannot find any information about its function and how t use it. If there is a problem with the systems BIOS might this be a path to a solution? Thanks in advance!