Exchange 2013 SP1 – The fully qualified domain name for node ‘DAG-EX2013’ could not be found.

To fix this:

1. Go to Active Directory Users and Computers
2. In ADUC be sure to turn on View > Advanced Features
3. Search for a computer object that represents your DAG. ie So if you name your DAG “Exchange 2013 DAG” you will have a computer object call this in AD.
4. Right Click > Properties
5. Under General tab, if you DNS name is empty, goto Attribute Editor tab
6. edit Attribute “dNSHostName” and add your FQDN of the DAG, so like “DAG-EX2013.YOUR.LOCAL.DOMAIN”
7. Hit OK and wait for AD replication. Then try to re-add the server to the DAG

You may also need to disable the object “<Your DAG Name>” in Active Directory if it's enabled. Otherwise you will get an error about it during adding of servers to DAG.

If everything goes well you should see success message that the move operation is complete.