[Cyclone III DEV KIT DK-DEV-3C120N] Flash Erase fails at 0x02900000

Does anyone have any idea why I can not erase the flash completely? I tried using Quartus II Programmer as well as with nios2 command shell. I tried different versions from 11.1 to 13.0. Erase in programmer does not complain and passes, but blank check fails at 0x02900000. And the address location where it fails to erase is always the same. Is there any logical explanation for this? Or is this the case of a bad sector?