Can you give us some more information on the error you are receiving when authenticating? Also check your error.log in App_Data->Sitefinity->Logs. What you can try is to use the web.config file from a newly created 6.0 project in your upgraded project and test if it will work.
I would advise to open a support ticket for this inquiry and provide us with all the details on your scenario in it. Feel free to share the outcome with the community afterwards.