I'm guessing that if you can login with FBA and you're getting this error, then probably the SQLMembershipProvider and/or database connection entries have been added to the SecurityTokenService, but weren't added to the web application you are accessing.
Check the Providers and Connection Strings sections in II7 for the web application.
To find the error in your SharePoint log file, i'll usually open the log file just after the error occurs, then search up from the bottom for the page name that was accessed. The exception should be nearby. Searching for "FBA Pack" will also find a lot of