SSIS Package - "Subquery returned more than 1 value" - Inserting one record into a table
In my case there was a trigger that trying to update one of the rows. The unfortunate thing was that the query in the trigger returned more than one row in the subquery.

As it turned out for me the trigger was only relevant for one particular supplier (an IF condition in the trigger), also luckily for me, the previous occupant of my position hadn't reviewed this trigger, and it was no longer needed.

Removed the trigger. However in other scenarios I would have had to modify the trigger to return one row only.