Subject TIB_LookupCombo: no selection possible
(yes, I know: way too many questions for a Friday afternoon...)

I am struggeling with TIB_LookupCombo - this is not related to my
other post.
I have a lookup combo to lookup a foreign key for a table. For some
reason, I ran into the situation where the lookup combo properly
displays the current entry when I scroll through the main dataset,
but when I try to change the selection in the combo, it shows the
list, but doesnt change the selection.
I dropped an edit field, bound to the same field in the main dataset
and it shows that I can enter the foreign key value manually (so the
column is editable).

What is wrong with the lookup combo? Two additional combos next to it
(which I am pretty sure are configured the same, just for another
field) work ok...