9 times of 10 when I see this it's a voicemail issue... Stop and then restart voicemail services from the RTC shell and try again.
Putty to port 2002/RAW and login with your system level credentials, then enter iPVM_Stop and wait for the command to finish, then use iPVM_Start to restart the services. Once it completes, logout of the RTC shell and try your backup again, most likely it will succeed.