This is typical for IIS 7 version and form controls which should do the postback. The solution is simple: you need to add/modify the following code to your web.config file:
<system.webServer>
...
<httpErrors existingResponse="PassThrough" />
<modules runAllManagedModulesForAllRequests="true">
...
</system.webServer>
Links to other resources: IIS 7 configuration
-it-