bummer, I misread the firmware upgrade posting in this group (the sticky one): I thought the 'upper page buttons' means 'button "up"' from the manual (
http://www.pocketbookreader.com/uplo...0_english_.pdf). But actually the top most 'pages button' is meant.
Thus pressing the the top most 'pages button' and the 'ok' button during power-on works as expected and the firmware upgrade was successfull.
Best regards