Yup - sometimes happy days, sometimes not.
I had the same problem come back again! So I did use scanpst this time, and it found and fixed errors.
But the sync failure continued.
Finally tracked down a few appointments that were causing problems by using the sync log again...deleted them and then re-accepted them and things have been fine since then (1 1/2 days).
Hopefully things will be stable again...the errors scanpst found may have been contributing to the problems, so am glad I ran it.