You are welcome.
For the rest of our visitors, this was caused by the ASP.NET and IIS misconfiguration. Whenever you start getting "500 internal server error" in your browser, it is wise to check MonoX logs (/MonoX/ApplicationData/Logs) and Windows event log. If both are empty, the browser request is probably not reaching the MonoX engine at all, and it may be solved by simply running aspnet_regiis -i from your .NET framework folder ( C:\Windows\Microsoft.NET\Framework\v4.0.30319 or something like that, depending on the version of the framework you are using). This will reinstall ASP.NET and update existing applications to use the correct version of the application pool.