I faced the same problem a few days ago and the solution is the same as Christian Graus said.
You could also try modifying your web.config to the following code:
<system.webserver>
<modules runallmanagedmodulesforallrequests="true" />
</system.webserver>
Remove the
<validation></validation>
tag and replace it with the
<modules></modules>
one in
<system.webserver>.</system.webserver>
Hope it works for you as well.