Perhaps you caught a problem with exchange server that I described few lines above? Can you get the logs? Just one doubt. Did as-daemon crash before you deleted the account? Well, in theory you might delete all calendars, then re-sync all of them... but let me think a bit more, probably there is something else can be done to fix those calendars. Btw, are you sure that it is "old" calendars and not a new one but with the same names and colors?