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
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.
Esri's BUG entry for "NIM065401 - Unable to edit a table within a personal geodatabase when it is a part of a composite relationship class." is unfortunately no longer publicly viewable.
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.