If you can't get the catalog backup working at NBU 5.1 MP2, you could manually do an equivalent catalog backup, like this:
1. Wait for a time period when there is no activity in your NetBackup environment (no backups, duplications, or restores).
2. Stop all of the NetBackup daemons on your master/media server by running the script "/opt/openv/netbackup/bin/goodies/bp.kill_all".
3. Manually use the Solaris "tar" command (or an equivalent utility) to backup the following directories to a separate tape or a flat tarball file:
/opt/openv/netbackup/db
/opt/openv/var
/opt/openv/volmgr/database
These are the same three directories that get backed up by NetBackup during a catalog backup. The first path contains your images and media databases, the second path contains your license keys (and a few other things), and the third path contains your volume database and the device configuration. If you choose to put this manual backup on a tape, pick a tape that you won't be using with NetBackup. Once a utility such as "tar" writes to the tape, it will destroy the NetBackup header on the tape and NetBackup will refuse to use it (unless you checked the "Overwrite tar" option in the media server properties, and then you'll run the risk of destroying your manual catalog backup with NetBackup backups).
At this point you should be in a good position to upgrade to NBU 5.1 MP7.
You should also be aware that NBU 5.1 reached it's "end-of-support" date back on March 31, 2008. Symantec is not producing any more patches or maintenance packs for NBU 5.1, and you may get the cold shoulder from Symantec tech support. I would bet that Symantec tech support will tell you to upgrade to NBU 6.0 or NBU 6.5.x. You may have to push back at them to get them to help you troubleshoot an issue on NBU 5.1.