Microsoft Exchange DAG Mail backup fails with v6 index migration error

For MS Exchange DAG mail level backup set created by AhsayOBM version 6, after upgrading the client application to version 7.3.2.0 - 7.5.0.2, when performing a MS Exchange DAG mail level backup, the following error message is received in the Backup Job Logs :

[YYYY/MM/DD hh:mm:ss] [info] Migrating index entry Job="Current" Path="Microsoft Exchange" Type="T" from v6 to v7
[YYYY/MM/DD hh:mm:ss] [erro] Unable to do backup, Error= [MigrateV6Index.migrateMultiHostIndices] Failed to deserialize v6 index
...
[YYYY/MM/DD hh:mm:ss] [info] Start running post-commands
[YYYY/MM/DD hh:mm:ss] [info] Finished running post-commands
[YYYY/MM/DD hh:mm:ss] [info] Deleting temporary file C:\Users\org-man\.temp\1449821630967\OBS@100
[YYYY/MM/DD hh:mm:ss] [erro] null
[YYYY/MM/DD hh:mm:ss] [erro] Backup completed with error(s)