I have two Exchange 2013 server setup in 2 sites. Both the sites having version Exchange 2013 CU6. Currently my ECP internal and external URL of site 1 is pointed to Internal server name of site1 and public URL. Second site, ECP pointed to internal server name and external to Public IP. When i Open ECP from second site, I am getting error message. This article refer the same issue and same setup. Does anybody having other solution apart from updating exchange 2013 CU7.