EXCHANGE 2007 Total backup issues

Hi,

I have a problem with backup for Exchange
Serveur Windows 2008 + Exchange 2007 SP2

planning for saving database every sunday total backup every day incremential backup
Sometime a the end of the total backup an error occur ( see attached file)
With a reboot of the server, we manually start the job it works
Can you tell us what's the meening of the error message

Regards