Server Error in ‘/’ Application when logging in using FBA

I have just extended a web application to allow users to login using FBA (Forms Based Authentication), however, when logging in the following error is generated:

Server Error in ‘/’ Application.

So, what could have caused this?  Firstly I checked that the SecurityTokenServiceApplication is functioning correctly by browsing to http://localhost:32843/SecurityTokenServiceApplication/securitytoken.svc on the SharePoint server – as long as the following message is displayed then that is OK:

The server was unable to process the request due to an internal error.

Next I checked all the connection strings and provider details (twice) and they all seemed to be correct.  After a bit of digging through the logs I realised that while adding .Net users and roles in IIS I had not set the Default Providers back to their original values.  Setting these back to ‘i’ and ‘c’ respectively solved the problem.