Thanks Frank, let me try and reply later today
This is a bit old, but anyway, you can always ‘reset’ bad blocks with the linux flash_erase --noskipbad command. Sure U-Boot can do the same with a different command.
Then try writing again. If it was marked bad for another reason then really a bad block (uncomplete unmount of ubifs or power cut maybe), it may work again.
Thanks for info, I will try it 
btw: NAND is working right now, even without UBI. When i wrote here about the bad blocks, I did not know that there is tolerance for bad blocks in NAND. Thanks for smarter people here, I learned a lot. 