Error retrieving the metadata for the data form instance: GUID: The request failed with HTTP status 401: Unauthorized.
I received the above error when I was trying to add a Data Form AddIn Spec into a Catalog project. When I selected that item the Visual Studio prompted me for my credentials after which it displayed a box to search for data forms. I found my data form, selected it and then received the above error.
I found an article on Microsoft's site that talked about issues that could occur with Windows Authentication and local loopback / DSN addresses. For my site and service base I had been using my DNS address and not the localhost loopback. Just to try it, I went in and changed this registry key to use localhost instead of the dns address:
Key - `HKEY_CURRENT_USERSoftwareBlackbaudAppFxSDKServiceUrlBasePath`
After changing this, I closed Visual Studio, restarted it, created a new catalog project and added in a new data form addin. This time, it made it through the process without throwing the 401 unauthorized error. Hopefully it doesn't create other issues. :D