It doesn't look like a Scheduler issue, but an RMAN one instead.
According to the error, the diskgroup space is exhausted, so perhaps you're running out of space on your disk?
problem could be with TAPE, please run RMAN script before schedule it.
I think problem not in Scheduler.Assume That problem with Tape or MML .First time run rman script and look at what will do this script .I think i t will be same error.please try it.
This is not scheduler issue but a RMAN issue. The backup failed to load MML for your tape.