Failed to submit job 'xxx'... Need entry in hookplugs.cfg to support this plugin (4032993)

Failed to submit job 'xxx'... Need entry in hookplugs.cfg to support this plugin

When running RMAN CLI restores allocating multiple channels, the NVBU Scheduler process (nvsched) will return the below error:

'Failed to submit job '%s' using plugin '%s' on client '%s'. Need entry in hookplugs.cfg to support this plugin'

Not all channels allocations result in a non-scheduled job and only few are able to run.
The restore job itself is not failed but will be run with less channels than intended.

When running RMAN CLI script to backup and restore, a NVBU non-scheduled job is created for each allocated channel of type SBT_TAPE.
For instance if an RMAN run {} statement specifies allocating 16 channels, 16 non scheduled jobs will be created by NVBU Scheduler.
The error is caused by the Scheduler process ability to handle simultaneous requests for job submissions.
Given the environment, CPU/Memory, load on the NVBU Scheduler, size of the NVBU databases, the simultaneous requests for job submission may be too much for the Scheduler to handle at a time.
On some sites the error starts appearing when only 2 or 3 or even 64 channels are requested.
The error does not indicate that the nvpluginaccess command must be run, as some channel do make it and create NVBU non-scheduled jobs.
Instead it is indicative that the Scheduler process could not process the incoming request.

There is no clear workaround for this issue, except that before running RMAN CLI script with multiple channels, one must ensure the Scheduler process (nvsched) is idle.
If possible run such script when no other jobs are running or are scheduled to start.
Also avoid running such script immediately after restarting the NVBU service as the Scheduler is known to perform ScheduleDatabase cleanup at startup.
Such cleanup is clearing non-scheduled job entries from the NetVault Database tables which is known to be CPU intensive and could prevent the nvsched process from
receiving messages hence checking their clearance and submiting jobs.