I had the same problem and added the NMM Exchange Admin user configured on the Networker client for the DAG.
- Open Networker Management Console
- Select client properties of Exchange DAG
- Select the Apps & Modules tab and add the NMM admin user created with NMM Exchange Admin Configuration
- Select the Globals (2 of 2) tab and add the FQDN of the same NMM admin user for your domain and the local Exchange servers (the client wizard config should have already added SYSTEM@exch1, SYSTEM@exch2, AND SYSTEM@exch3)
Maybe a bit overkill, but I was tired of messing around with it and just needed the backups to work.