Subject Where Clause in LookupCombo causes Multiple rows in singleton select error
I have a query with a few "normal" fields plus two IDs which are populated using TIB_LookupCombos.
At first, both LookupCombos simply selected all data from the underlying table and everything worked fine.
Then I had to limit the data in the second LookupCombo based on the selection of the first LookupCombo.
I did so by using the AfterScroll event of the first LookupCombo and adding a where clause to the query of the second LookupCombo.

The actual queries are difficult to read, so here in pseudocode what the two queries look like:

Select ID, Supplier from TableSupplier

Select B.ID, B.Branch, B2S.SupplierID
from TableBranches B
join Branches_X_Supplier B2S on B.ID=B2S.BranchID
where B2S.SupplierID=:Supplier_ID

Now, as soon as the the where condition is set, I get a Multiple rows on singleton select error.

What I don't understand, with the where condition in place, I get much less data than w/o it and still get the error.

Any hint on how to find the culprit would by very welcome.