There is size limits for database backup in SEPM.
I have seen scenarios where restoration of a huge backup files failed due to restriction of the database size on the SQL database of the new SEPM (on which you are restoring the backup). In such cases, just setting the size limitation for the database (in SQL management studio) to "unrestricted growth" and then restoring the database again has always worked.
http://www.symantec.com/docs/TECH184770