I had the same problem with our Tera Station when we first set it up.
if you go back into the network settings and to Workgroup/Domain
the first thing it asks for is Active Directory Domain Name
(NET bios Name) when you put this in just put in your domain name and not the .blah bit
ie domainname and not domainname.local
that should do the trick
but make sure the second option i.e. the AD Domain name (DNS/Realm) hase the full domain.local
thats what i found anyway and ours is a TS III ES 8TB