I am writing an article about creating setups for Visual Studio add-ins and testing I have found that if the namespace/class specified in the <FullClassName> tag of the .AddIn XML file does not match the actual namespace and connect class name in the source code, you get an obscure <Unknown error> (error number 80131522) loading the add-in. Searching the web I found in the forums that I was not the first developer with this problem, so I have updated my article to reflect this:

HOWTO: Troubleshooting Visual Studio and Office add-ins
http://www.mztools.com/articles/2007/MZ2007009.aspx

It would be great if Microsoft could provide more helpful message errors when things are not set up as expected…