Display goes dark during boot

Have been using urbackup for a long time, never seen this behavior:

  • HP ProDesk Mini G3
  • recovery cd 2.4.2
  • boots successfully (tried both, UEFI and legacy)
  • after showing some lines of boot messages, the display goes dark (no signal)
  • display is connected via HDMI, I tried 2 different monitors and cables, same behaviour
  • failsafe mode gets stuck as well (after line „probing EDD“ … ok)

Any ideas what could go wrong?