When troubleshooting database backups, it is important to remember that a database backup is in principle an user initiated backup,
so the backup is actually started from the client by the script/database backup api.
The database backup is more sensitive to name resolution than file backup, partly because of the above
Places to look for information/things to do:
The activity monitor, probably where you would see the error first.
The bphdb log folder which gives the script exit status, environment information and other things there can help figure out what the issue is.
The dbclient log folder with 777 permission which gives detailed information about the backup done by the database api e.g. rman
The database api's log e.g. rman messagelog
The database's log e.g the alert log for Oracle
The OS system logs, but often this a copy of the messages from the database's logs
If none of this gives an indication of the problem run the script from a prompt if possible, as this might give an output not catched by any of the logs
In case of layered scripts like Oracle, first run the cmd/shell script as administrator/root,
then run the rman script as administrator/oracle user
Sometimes it is necessary to run the scripts one line of time to catch the error
In my experience this approach enables you to find most of the issues with the database backups