update. after completely draining the battery, the cmos seems to be cleared and i could enter the bios, and also boot up windows properly.
not sure that the cuplrit was though.
many thanks to the F7 owners on this forum who tipped about the battery drain. since F5 seems to automatically power down when the battery is low. ( the typical battery icon with low level in red appears) i had to plug the charger in for a brief moment and then unplug it. repeated this several times.