MichaelH I found 2 sets of updates today for my appliance (15-04 ISO). First was available by 09:30ish with another update made available sometime before 12:15. The 2nd update contained the fix Gromox 2.28.40.g0eb259b-lp155.44.1 | x86_64 that should have fixed the issue caused by Saturday's update.
However, I have found that whilst the fix appears to have resolved my issues with Mac Mail, my iPhones, whilst now syncing OK, have been reporting an Account error since Saturday.