Causes:

MMS Application Server and Backup Daemon are installed on different machines. And these two machines do not share the gen.key file.

Solutions:

On the MMS Application Server, run the following:

grep ENC_KEY /opt/mongodb/mms/conf/mms.conf

On the Backup Daemon Server:

grep ENC_KEY /opt/mongodb/mms-backup-daemon/conf/daemon.conf

If the gen.key file is missing on Backup Daemon Server or gen.key are different on the two machines, copy gen.key from MMS Application Server to Backup Daemon Server and restart Backup Daemon:

sudo service mongodb-mms-backup-daemon restart