After a migration from Exchange 2007, backups fail intermittently
Article ID: EXMB0003 Backups in an Exchange 2010 or later environment fail intermittently after a migration from an Exchange 2007 environment
After you migrate from an Exchange 2007 to an Exchange 2010 or later environment, and then decommission the Exchange 2007 server, mailbox backups fail intermittently.
The following error message might appear in the ExMBBackup.log file on the computer where the agent is installed. The log file is located in software_installation_path\Log Files.
In some cases, the Public Folder references still point to legacy Public Folders even though the Public Folder database was deleted.
Use the ADSI Edit snap-in to change the msExchHomePublicMDB value to not set as described in the Resolution (Method 1) of the Microsoft support article "Outlook clients repeatedly disconnect from and reconnect to Exchange Server 2013": https://support.microsoft.com/en-us/kb/2962915.