I have this issue when running backup for my Oracle cluster server, I'm using Lan-free backup via SVTL raw device to the Oracle client server and only getting this while running the job.
NetVault Support is going to need to see the full logs to assess what is going on here, because all I can really tell from this is it has a failure allocating the channel. What version of RMAN and what version of the Oracle Plugin are you running? Your best bet will be to open a support ticket on support.quest.com.