Try putting the ags domain account into the servers administrators group.
There are a number of bugs that Esri reports which address similar behavior, but since you are at 10.2.1 I'm thinking this is a different issue entirely. What does Fiddler show when you request that URL?
Have you tried opening an incident with ESRI technical support?
Got this same error today 28/04/2014 while using AGS 10.2.2 on windows server 2012 R2. Tips from this forum helped me fix the issue but am afraid i had to use http://localhost:6080/arcgis/admin to join the machine to site. My steps were as follows. Removed all references. created a service account named "arcgis" in the AD Gave the account ulims\ArcGIS user rights on server used the domain account o install ArcGIS server. received the please wait.... hang screen. used admin to join the server to a site. See logs on the same. http://1drv.ms/1GCRITp Now it works.
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