15.x has a bug with dbComboforms.

It's not mentioned in the Help, but I assume the Entry_State is True by default?
The properties panel however, shows False as the default value.
When you compile on the other hand, the Entry_State is True and you can enter the dbComboform.

This all can be seen in the OrderEntry sample.

So in order the set the Entry_State to False, you can't use the properties panel, but you have to manually type the line 'Set Entry_State to False' otherwise it won't work.