This error will occur for many reasons. If you've arrived at this blog, however, the odds are that you're searching for something .NET-related, possibly even a provider-related issue.

A .NET provider specified in a web.config file has its status set to enabled=false by default. Don't ask me why.

This problem, when using a custom role provider, might manifest as the above error. Very frustrating, as none of the Google results I've seen will mention this particular cause of this error, anything about how to actually fix it.