Hi,
apparently something goes wrong but at the moment you don't get to see the real exception. Just do as this message tells you and change your web.config file to have this in it at the correct place:
<customErrors mode="Off"/>
Mind that this is case sensitive so it should be written exactly like this.
Then upload the web.config file again and request your page/file again to get a much more detailed explanation of what goes wrong.
Grz, Kris.