I'm sure it happened because i had it working for all the objects before upgrade.
I wonder was this actually to do with the appointmnet object and not the upgrade.
I don't want to use the required start / end fields in my system as they are tomestamp and I need only date. Also Subject is to be populated automatically. So I had all three of these off the form and being updated by rules. I noticed prior to the upgrade that when I was creating entries for some BO's, it would give an error that the Start wasn't entered even though I had a rule for it. When I checked, the rule was gone. I suspected I had just over-looked it for that BO but I am sure it happened again at a later stage even when I had corrected it.
My solution now is that I have populated the necessary fields as part of the EnterNew process and this seems to be fine. So maybe the appointment type objects do not like rules to add values to these fields when it is a New Record ?