Why do I keep getting WebDAV.facebook.com errors?

every time I turn on either my iPhone or my iPad I get a message 'cannot verify server identity' because WebDAV.facebook.com cannot be verified by Facebook. I then have to manually approve each certificate - I get four or five every ti e

why is this? How can I stop it? Is it because I am in China & so cannot access FB directly?

its driving me crazy, appreciate any help

cheers

Ian