I have 5-6 different copies of WP in separate directories on a dedicated machine. I serve them via separate virtual servers in apache.
When I restart apache, any one of the WP installs will work, but once one is used the others won't return anything, not even a response.
The web servers are working, if I have other content on the domain besides WP it works fine. When you request a WP url it's like it never happened. Nothing in the access_log, error_log, nothing.