This to me is authentication issues between the accounts used for the isilon and filer.
These may need to be local accounts on both devices and see if that works. if you are using AD then ensure that the necessary RBAC polices are in place.
I also suspect that you need to setup the ndmp daemon authentcation on the FAS correctly. options ndmp also run ndmpd status to see what is going on. check the messages files in /etc/ to see also for errors. Also make sure the ndmp authentication is plain text initially to test.
Started with basic troubleshooting and figured out at very first step, ndmp wasn't enabled on filer. Enabled and changed authentication to "Plaintext" works smooth not... Just curious do we have any reverse migration tool as well here ?