I found out the problem already, thanks for your response.
The TableX does not have a primary key, only a unique constraint on the two fields.
Deleting with <entity>.Delete() doesn't work then, since the record to be deleted can not be specified based on the info within the entity.
Using the <collection>.DeleteMulti(filter) ... does work if the filter specified corresponds with the conditions as specified using the FetchUsingUCAidBid(valueA, valueB)
So ... I'm happy again ... ;-)
I am running into a new problem though ... something with a left join which doesnot seem to work properly ... or is it me again?? I'll start another thread on that.
Paul