Hi Tom,
At this stage I really cannot think of anything specific. It is a known potential cause for such a problem that new version's designer is writing a slightly different code, compared to an earlier version's designer, where you originally created or even use to modify the form at some time.
If there is some missing code in the serialization, you can usually "fix" it by opening the form in the new version's designer and then save it back. That should serialize the missing designer code.
On the other hand, if there is an excess setting, that setting would stick through open-save procedure.
Palli