Mailbox backup may fail with error:
"Processing mailbox %firstname.lastname@example.org% 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"
Processing mailbox: email@example.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.
at Veeam.Archiver.Source.SyncPage.SaveChanges(IFolderRepository repository)
at Veeam.Archiver.Source.ItemProcessor.ProcessFolder(Byte id, ExFolder folder, Byte currentState)
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: firstname.lastname@example.org