Thanks, I have been asking Symantec how large a timeout I might expect and have not received a clear answer. It is helpful to know that it may simply be a timeout issue.
The system has between 1 and 5 million files at any given time, that is the reason I want to use Flashbackup. The regular file backups take up to 18h to complete.
I increased the timeouts on the master, media and client server.
It is odd though, that the first time I attempted to backup this volume it succeeded with the timeout set to only 300 and everytime since has failed even with the timeout set much larger.