1521 - Error: "Content stream ... could not be retrieved" (M365 Exchange or OneDrive)

Symptom

A backup for Microsoft 365 Exchange or OneDrive completes with errors. The message below is found in AppInsights:

Content stream for Mailbox\Migrated Teams Chat\AAMkAGRhMjRlNWY1LTgxZjktNDVmOC1hYmExLTA2MWJkMT
fg+AAA=_000 could not be retrieved

Cause

This error is probably the result of a document being open on a user's machine when the backup starts. This means that the data for that file cannot be collected during the backup process.

The error can also present if the file is moved or deleted while the backup is in progress.

Solution

This error requires no urgent user action, as the affected files will simply be queued for data collection during the account's next backup.

The only way to prevent this error is for all files to be closed at the time of backup, which may not be feasible if the backup takes place during office hours. One alternative might be to change the backup time to be outside of office hours.

If this error is logged for the same file for multiple consecutive days, our Support team will need to investigate further.