Unable to edit a table within a personal geodatabase, when it is a part of a composite relationship class.

Problem:

If a table is part of a Composite Relationship, it cannot be edited on a MDB. Following
error message is returned.

Objects in this class cannot be updated outside an edit session

Solution:
This is a known issue in ArcGIS. The behavior only occurs on a MDB and if a Unique
Index and a Composite Relationship is used.

http://support.esri.com/en/bugs/nimbus/TklNMDY1NDAx

The usage of Unique Indexes is required on an MDB. The only option is the use of simple relationship classes.

If this is not possible, but the corresponding configurations are still made on an MDB, the configuration must first be transferred to another database system in order to test the configuration.