Stuck in a sign in loop on a new install DS918+

So:
DS918+
Proc: INTEL Celeron J3455
Mem: 4GB (extra stick still in the mail)
Ver: DSM 6.1.3-15152 Update 5
Plex: 1.9.2.4285

1. Package Center says installed correctly.
2. Plex user created, access to plex and video folders confirmed.
3. Active account on Plex and can log in and see account on apps.plex.tv
4. attempt to log in via the http://xxx.xxx.xxx.xxx:3200 and I get this:
   
5. Click Sign In and I see the start of the normal desktop and then it immediately reverts back to the above image.

Ideas?