what are the basic reasons for any database backups failing with error 6

[Andy_Welburn]

For example:

STATUS CODE 6: Veritas NetBackup (tm) for Oracle database backups fail with a NetBackup Status Code ...

The SQL Server database backup exits with a Status 6 in Activity Monitor when using the NetBackup SQ...

etc etc etc

Your best bet would be to look through the following for Status Code 6:
*****************************************************************************************************

This Technote contains a table for NetBackup's Status Codes, with links to potential resolutions for...

[rjrumfelt]

Is pretty much the gist of this error code.  If you could be a little more specific, such as a specific type of database backup, it would be easier for us to point you in the right direction.