I've got the iDisk application installed, and I saw this error message when I synced my phone this morning. I tapped OmniFocus' sync button again, though, and the sync completed just fine.
Just browsed my iDisk from the phone and was still able to sync OmniFocus. It looks like a one-time glitch to me (at least so far).
If you're still not able to sync, the steps from
this thread should work just fine.