error opening the snapshot disks using given transport mode status 23 message on a single vm

i'm running netbackup 7.6.0.3 with 5220 appliance.

this message only appears for one vm backup.  I can backup another vm on the same datastore without a problem, so it's not an issue for the appliance to see the lun.

I can also backup a vm in the same policy.  what else can I look into?  I added nbd to the policy and it worked.  I don't see any issues with the name or dns.

thanks,

rudif