There is a limitation to the number of records that can be passed into IRelationshipClass2::GetObjectsMatchingObjectSetEx when the relationship class is based on a GUID field.
I can think of a couple of workarounds:- Loop through an enumeration of the source records. For each record, call GetObjectsRelatedToObject or do a QueryFilter based on the foreign key to the related table. This will probably be slower than GetObjectsMatchingObjectSetEx.- Convert the GUID field to a text field using the VBA function cstr(). This will not be a good work around for users that want to stick with GUIDs.