Just ran into this same issue. Was able to join by using... DOMAIN\USER and the password.
I am trying to join our Storage Center to our domain.  I have the configuration correct and when I run "Test Servers" it completes successfully.  I then attempt to join with a domain admin username/pass, but it continues to fail with:  Summary: Join domain failed for host: Cause: Join domain failed for host:  Very vague detail.  Has anyone seen activity like this?  
Storage Center & Joining Directory Services