Event 9399: OALGen is configured to generate version 4 OAB files for offline address book ‚\Default Offline Address List‘ and publish it to public folders, but there is no public folder server available. OAB version 4 will be generated but will not be published to a public folder at this time. Please ensure that a public folder server with a replica of the Offline Address Book system folder is online and mounted, or disable publishing OAB version 4 to public folders.

The Generation Server for the Offline Address Book must have at least one online Mailboxdatabase. In a DAG you should never locate it on a node which will hold the passive Databases – it will result in the Error above. Just move to the Active Server ( EMC: Microsoft Exchange – Microsoft Exchange On-Premises – organization Configuration – Mailbox – Offline Address Book )