Mailbox backup may fail with error:
"Processing mailbox %email@example.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: firstname.lastname@example.org... 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: email@example.com
If you running version 18.104.22.1680 or older, the issue has been resolved in Veeam Backup for Microsoft Office 365 1.0 Update 1.
If you running version 22.214.171.1242 or newer, contact Veeam support directly to obtain the fix.