When creating a custom defined table, the SchemaExtension tool creates a DialogObject automatically. The DialogObject is assigned to the change label as well.
After that, defining a DialogSheet (a form for that table for the Manager) results in the creation of two new objects: DialogSheet and DialogObjectHasSheet.
When creating a transport package for the schema extension it puts the DialogObject into the package and, if the option "Add dependent objects" is active, it adds the DialogObjectHasSheet as well, but not the DialogSheet.
When the Transporter tries to create the DialogObjectHasSheet it needs to have the objects of both sides of the relation - but the DialogSheet is missing, hence the error.