Ahh, Thank you. Gearbox really seems to be very likely the problem. I don't think that seized engine would make such horrible noises. Damn, bike is not so old 4 years and have 20000 on the clock. Engine is pretty weak so stress on the components should not be so big and I 'm riding it carefully and slow like grandma, changing gears properly and doing maintenance every week.
How that can happen?