Firstly, I'm assuming all design elements in the application are signed with the same ID? It might be this one page has been modified recently, but not by an ID with signer rights possibly?
Secondly, are you certain the server is up as single server? This sounds similar to an issue I have had in the past where the server was configured for Multiple Servers (SSO)
Try using the fully qualified internet host name of the server in the URL. This should work if its set to multiple servers.......