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In addition, following Andrew's original post at http://www.andrewconnell.com/blog/articles/HowToConfigPublishingSiteWithDualAuthProvidersAndAnonAccess.aspx on a step by step basis, may not be sufficient.
On an exsiting Farm which was created and configured using Kerberos Authentication as per Microsoft's best practices (and with applications working perfectly using Kerberos Authentication), we had experienced three failed attempts at accomplishing this, while following the original article to the letter. The process has been repeated three times, with three failed login attempts.
On entering the login information for a user that had been created using the SQL Membership Provider, the user is presented with a SharePoint "unknown error".
As the error is a SharePoint page, not an IIS error page, this tells us that there is a problem with the configuration of either the web.config files, or the Central Admin settings.
The user which the application pools for the new FBA Application are running under needs to be added to the FBA Database permissions.
After adding the user, everything should works as expected.
We're running FBA for external access in an all Kerberos environment. While this does take some additional work on the domain controller via setting Service Policy Names, and raising the domain functional level to 2003, the result is a faster and more secure SharePoint.