Internal Server Error 500 for just One subdomain for users not logged in

We are running a wordpress multisite install successfully since 2 years.

Recently we noticed that one of our subdomains shows alright for users who are logged in, but throws internal server error 500 for users not logged in. The subdomain is:

We have tried disabling all plugins and changing the theme but the behaviour does not change. The subdomain comes up alright if the user is logged in but gives 500 internal server error to users who are not logged in. All other subdomains like,,, etc donot show this behaviour.

Please help us resolve this issue. Any ideas about what could cause this?