By default, IIS7 blocks error messages from being viewed remotely.
This also applies to PHP sites such as WordPress hosted under IIS7 which by default might display a 500 error without the following config changes.
To enable the viewing of error messages add the following line to the <configuration> section of your web.config file:
<system.webServer>
<httpErrors errorMode="Detailed" />
</system.webServer>
If you don't currently have a web.config file it should look like the following:
<configuration>
<system.webServer>
<httpErrors errorMode="Detailed" />
</system.webServer>
</configuration>