Microsoft Exchange Backup Errors "No buffer space available"

When performing a MS Exchange mail level backup (both Exchange server and Office 365 mail level backup set), the following info message is displayed in the backup client log:

[YYYY/MM/DD hh:mm:ss] [info] [EwsService.syncItems] Retry after 20 seconds, count #1. Error: [g.a] Retry: 1420, exception: java.net.SocketException: No buffer space available (maximum connections reached?): connect

Note:
When the error occurs, the backup user cannot login to the AhsayOBM client and CPU usage on the client computer remains high during the backup.

MS Exchange server backup works fine for the Exchange setup.