Anyone know why SMF->WordPress integration would stop working?

It's not something I've done myself. It happened to ALL of my SMF sites at once including one which I haven't touched in over six months. They're all hosted on the same service so presumably something on the server has messed it up.

It's the ssi_login and ssi_logout which aren't working. Even though I'm logged in, the site still shows a login box

Any ideas? I'm trying to move hosts right now, but I'd much rather just stay where I am if I can figure out why this has happened.