Set field by name needs the first parameter to evaluate to: tableoccurrencename::FieldName.
Get (ActiveFieldName ) only returns the text to the right of the ::.
Set Variable ( $$filterTO; Value: Get ( ActiveFieldTableName ) ]
and then you can use this set field by name step:
Set Field By Name [ $$filterTO & "::" & $$filterFieldName; $$filterFieldValue ]
Perfect. Thank you!