Error "Object reference not set to an instance of an object" during mailbox backup with Veeam Backup for Microsoft Office 365

Challenge

Mailbox backup may fail with error:
"Processing mailbox %mailbox@name.com% failed with error: Object reference not set to an instance of an object."

You will also find these entries in "C:\ProgramData\Veeam\Backup365\Logs\Veeam.Archiver.Service_%date_time%.log" log:

Processing mailbox: mailbox@name.com...
Syncing folder items: name...
Sync time: 0.562066
Changed items: 26, deleted items: 0
Estimated changed items size: 1.77 MB
Error: Object reference not set to an instance of an object.
Stack:
   at Veeam.Archiver.Source.SyncPage.SaveChanges(IFolderRepository repository)
   at Veeam.Archiver.Source.ItemProcessor.ProcessFolder(Byte[] id, ExFolder folder, Byte[] currentState)
   at Veeam.Archiver.Source.ItemProcessor.<>c__DisplayClass6_0.<Process>b__0()
   at Veeam.Archiver.Source.FolderRetry.<>c__DisplayClass14_0`1.<Process>b__0()
   at Veeam.Archiver.Source.FolderRetry.Process(Action action)
   at Veeam.Archiver.Source.FolderRetry.Process[T](Func`1 func)
   at Veeam.Archiver.Source.ItemProcessor.Process(ExFolder folder)
   at Veeam.Archiver.Source.FolderProcessor.DoMailboxWork(CancellationToken cancel)
   at Veeam.Archiver.Source.MailboxAction.DoWork(CancellationToken cancel)
Mailbox processing failed: mailbox@name.com

Solution

We are aware of the issue and currently working on resolution. Once available, hotfix and implementation steps will be attached to this article.