I'm also getting an error when trying to register it, but it's a bit different (see attached image). I have an open ticket with ESRI, but no useful information yet.
The adaptor does appear in the adaptor list in the Manager webapp, but gives me a different (proxy-related) error whenever I try to access the REST directory with it.
I'm using Linux and deploying the adaptor to Tomcat. The web adaptor worked fine at 10.1 final, but can't even be registered properly at SP1.
After installing SP1 on both Server and Web Adaptor, I'm unable to register the Web Adaptor with Sever. I get the error:
Application Error
Unable to register the GIS server with the Web Adaptor. Please make sure that that the GIS server and WebAdaptor are of the same version.
Has anyone seen this issue?