Veeam Office365 Backup: Mailbox Processing Errors

Have been doing some testing with Veeam’s new Office365 Backup solution, so far pretty impressed. Did run into an interesting issue out of the box, we had some random mailbox processing errors such as:
– Failed to synchronize item changes. The operation has timed out.
– Async batch export failed with timeout.
– Mail item data export failed. There is an error in XML document.

Originally I just attributed this to “issues in the cloud” with EWS since Veeam uses this but after a day or so it became clear something else was happening here; as a safety net did some packet tracing in our firewall cluster to rule out potential security services causing the timeout. I did some additional testing and also spoke with Veeam support, their recommendation was to add the following line immediately after the <Archiver> tag in config.xml located in: C:\ProgramData\Veeam\Backup365

<Source WorkerThreads=”4″ BatchSize=”10″ BatchPart=”10″ BatchTimeout=”900″ BatchMaxItemSize=”5″ />

After adding this line and re-running the job it seems to have resolved the issue as all of our mailboxes are now backing up! Currently this issue was happening on build