In 6.5 the following worked:
With an UDF for item data (<i>OITM.U_Cat1</i>):
- add a field to the "<i>item data</i>" system form
- bind the field to <i>oitm.U_Cat1</i>
- use the field without additional code
This was done to permanently "mount" some of the UDFs into the item data form. It worked on 6.5, but with 6.7 the field value isn't written to the database anymore if I update an item. I can still use the UDF panel, but this is not what the customer needs.
Well, well, I changed a lot in the code and now it works. Unfortunately I can't exactly proove or even tell what it might have been; I'm too busy trying to migrate a fairly complex plugin to 6.7 and I only have got one day left...
Anyway, there's still a significant and yet undocumented delta between 6.5 and 6.7. Migration is a sure issue for hours of limitless fun.
Sorry everybody; issue closed from my point of view.