503 error on directory page

My client is experiencing issues with the 4th page of their business directory. When you click on the “Next” button to view that page, they get this message:

Service Unavailable
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

Additionally, a 503 Service Unavailable error was encountered while trying to use an ErrorDocument to handle the request.

Is this a php issue? Or something wrong with the plugin? Please let me know

Thanks

The page I need help with: [log in to see the link]