After some experiments I can tell that my problem is solved with removing the 8Gb memory card from the slot. With the empty slot N900 boots flawless and then I just insert the card back in slot and everything works fine as far as I can see.

Right now I switched off my N900... now I'm switching it back on...no luck.

Ok, I'm removing the batt and the memory card...switching on...booting...pin code correct...nokia tune....booted. Now trying to reboot...switched off...back on...pin code...tune. It works.