Unable to add Child Domain to AADSync

Hi,

I'm trying to add a child domain to our AADSync and when I modify to containers to sync, I get prompted to save the connection and when I do, I get:



Event log shows:

The account is a member of Enterprise Admins and I can modify all facets of AD.  There are obviously groups/users/accounts in the child domain.

Any help would be greatly appreciated.

Thanks,

Tim.