OLT MA5600 from RO16 to R015

I receive error when downgrading. The same soft files work on before downgrade procedure.

MA5600T(config)#load packetfile smart ftp x.x.x.x MA5600V800R015C10_patched.bin

{ <cr>|user<K> }

:

Command:
load packetfile smart ftp x.x.x.x MA5600V800R015C10_patched.bin
The new packet file will overwrite the old one
Rollback function will be disabled
Be sure that the system needn't this function
Are you sure to load new packet file? (y/n)[n]:y
Info: Standby board is abnormal, and command of loading for standby board is impossible
Load(backup,duplicate,...) begins, please wait and notice the rate of progress Any operation such as reboot or switchover will cause failure and
unpredictable result
The loading starts
PARAMETERS :FrameID: 0, SlotID: 10, Position: -1, Load type: Packet file, Load Object: Active control board

MA5600T(config)#

MA5600T(config)#
The loading fails
PARAMETERS :FrameID: 0, SlotID: 10, Position: -1, Load type: Packet file, Load Object: Active control board, Failure cause: The packet file is incorrect

Thank you :)