This is not the security issue.

How it was found

Here a subdomain

path: https://apps.owncloud.com/ a strange thing in this domain is that a new user can not create account but he can login in.

On that path:- https://apps.owncloud.com/usermanager/new.php

In this path registration file has been removed or something and saying 404 error. I mean why?

Okay for more informative i am sharing a screenshot please have a look.