You probably unplugged the one interface that is used to resolve the DD hostname by your backup application.
There are basically 2 options for you;
1: As said above, resolve to the DD hostname as your backup target device onto a failover or LACP interface.
This IF doesn't need to be part of the ifgroup membership, it will just be the initial communication and then appropriate DDBoost ifgroup IP will be handed back to the host for backup use and the hostname is no longer required for the backup.
This allows both resilience for the initial connection and incase one of your ifgroup interfaces goes down, also load balancing. The ifgroups do not need to resolve, they just need an IP.
If you cannot create a resilient interface with failover or LACP then you can add a hosts file entry at the client for the DD "hostname" on one IP/NIC and a second IP/NIC with the DD "hostname-failover" hostname, if the first hostname/IP is down, it will connect to the "hostname-failover" IP/interface and then discuss which ifgroup IP to use for the backup (as said above).
It's discussed in here -->
Hope that helps.