The customer is getting the error on the login page that says: The server does not have a valid license file.

This means that the customer has a limited license and has used up all of the slots. One quick way to use up all of the licenses is OSX users using WebDAV connections. OSX keeps opening new connections and does not close out the old one and thus webNetwork would have to wait until the session times out before releasing the user's connection. 

The admin can use the webMonitor application to force some of the connections to close out to get things working again.