Change the following values in your web.config file so that you can see what the actual error is: 

1.     Change CallStack="false" to   CallStack="true"

2.     Change <customErrors mode="On"/>  to   <customErrors mode="Off"/> 

3.     Change <compilation batch="false" debug="false">  to   <compilation batch="false" debug="true"> 

When you are done, make sure to change these three back to its original value as there is a performance hit when these are turned on/enabled.