Iam not in a position to answer what for the exception occurs unless and until I am able to see and validate the file.
Eventhough try out as I suggest:-
Always before importing the component, use the following command to check whether the file is proper or not
usage:
ipsadmin -chkxml <file-name.xml>
One way I can just you that try to get the iwtAuthMemebership component from the profile server using ipsadmin command, copy the content and develop your AuthMemebership component by changing the required attributes. Save your file then check the xml file and then import the file.
If you find any difficulty let me know.