You are probably hitting the following issue covered in this kb.
Issues configuring the backup program backups are failing:
https://kb.netapp.com/Advice_and_Troubleshooting/Data_Protection_and_Security/NDMP/Issues_configuring_the_backup_program_backups_are_failing
To resolve this issue, For the "preferred-interface-role", you can opt for 'data'. But, please bear in mind especially for 'Data' LIF, the order in which it is placed in the 'ifconfig -a' output is a key, unless you have configured a separate dedicated VLAN for NDMP backup traffic. Please have a look at the following kb for detailed information.
What is the LIF Choice order for Cluster Aware backups in NDMP?
https://kb.netapp.com/Advice_and_Troubleshooting/Data_Storage_Software/ONTAP_OS/What_is_the_LIF_Choice_order_for_Cluster_Aware_backups_in_NDMP
On the side note:
As I have never done NDMP backup (never had an opportunity) of flexgroups, so I am not sure if all the principles that applies to flexvol applies to it. In anycase, please go ahead and do the testing.
How are NDMP backup logs of FlexGroup different than FlexVol logs?
https://kb.netapp.com/Advice_and_Troubleshooting/Data_Protection_and_Security/NDMP/How_are_NDMP_backup_logs_of_FlexGroup__different_than_FlexVol_logs%3F
https://docs.netapp.com/us-en/ontap/tape-backup/ndmp-management-flexgroup-concept.html