I was setting up the connector for VSTS and Project Server (http://www.codeplex.com/pstfsconnector), and ran into a couple errors, including the title of this post. Luckily, the connector does some excellent logging, just not by default.
In the web.config file for the app, which is at C:\Program Files\Microsoft Project Server 2007 Connector\Connector Web Site by default, find the following:
<loggingConfiguration name=”Logging Application Block” tracingEnabled=”true” defaultCategory=”General” logWarningsWhenNoCategoriesMatch=”true”>
<add source=”Enterprise Library Logging” formatter=”Text Formatter” log=”Application” machineName=””
And change the tracingEnabled to “true”.
Then, just rerun the web page to get the error again, and then check your event log (as in, admin tools à event log)