The fragment version was my first idea also: checked them all and ensured their all in the correct version. Without any help.
I guess this is more like a 'feature thing'. I tried to rebuilt the form from scratch and found already some issues:
1. It seems Acrobat/Reader 7 don't like fragments on Master Pages. That starts LC Designer to complain about this "exceeding capability"
2. This is a strange one: as soon as I add a (regular) button in a Master Page, this message appears also. Strange, because this shouldn't be a problem! It works properly in a blank form without other items in the header, but at soon as I add the style items this problem appears. (The items on the Master Page are: 2 (positioned) subforms, an (embedded) image, a line and 4 text-objects.)
Any ideas?
(Strange enough Adobe has some whitepapers about differences between target versions, but there is nothing about fragments nor restrictions on Master Pages in it.)