Are you having the same issue with logging in via the standard Joomla login module? I would suggest trying that to see whether it is affecting all authentication or not.
Next, I would try setting the Joomla Login Redirection setting in SCLogin to a different value. It's possible that it's pointing to a page that is inaccessible to the current user's group (such as the menu item being pointed at is Special, etc)
Try these two suggestions and let us know if you still have problems.
-Melissa
Problem with Login
