It recently dawned on me that the issue I was facing was due to my system.webServer having handlers for ServiceStack that were grabbing all content:
<handlers><add path="*" name="ServiceStack.Factory" type="ServiceStack.HttpHandlerFactory, ServiceStack" verb="*" preCondition="integratedMode" resourceType="Unspecified" allowPathInfo="true" /></handlers>
Additionally, I had included staticContent to handle the JSON file extension in Windows8 IIS:
<staticContent>
<remove fileExtension=".json" />
<mimeMap fileExtension=".json" mimeType="application/javascript" /></staticContent>
Upon removing the staticContent, I encountered a 403 error. However, upon re-adding it, the ServiceStack handler took precedence and resulted in a 500 error. Could this be a similar issue for you? Ensure that your configuration is properly set up to handle the mimetype.