Mit der Fehlermeldung konnte ich nicht wirklich was anfangen und hab erstmal ewig rumgedoktort… Hier die Lösung/Erklärung:

RMAN-06091 no channel allocated for maintenance (of an appropriate type)

Example

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of delete command at 06/03/2008 23:22:20
RMAN-06091: no channel allocated for maintenance (of an appropriate type)

Details

Backups failing due to the following when running any of the following options with “delete obsolete”

RMAN> delete obsolete;
RMAN> delete noprompt obsolete;
RMAN> delete force obsolete;

Solution

Run the following to specify the device type , in this example use device type disk:

RMAN> delete noprompt obsolete device type disk;

If the above fails, allocate the appropriate channel for maintenance , perform the appropriate crosschecks and delete option.
In this example the device type disk was allocated for maintenance

ALLOCATE CHANNEL FOR MAINTENANCE DEVICE TYPE DISK;
run
{
report obsolete;
CROSSCHECK BACKUP;
CROSSCHECK COPY;
DELETE EXPIRED BACKUP;
DELETE EXPIRED COPY;
delete obsolete;
}
release channel;