Did you ensures any logger capability in your WEB App??
First thing i can advise to you log any error that arrives in method
protected void Application_Error(object sender, EventArgs e)
To override this method open global.asax and do method declaration.
As core of log system you can leverage either log4net or NLog third-party assemblies.
This might be helpful for U.
Another thought that came to my mind,
Do you use a third-party libraries in you web solution ?
if yes , so, in such case maybe you forgot to deploy it to remote server??