Symptoms

The error appears during o365 SharePoint backup.

Cause

1. The user created the lists is been deleted completely;

2. The behavior occurs if there is something wrong with the users record within the User Information List for the site collection;

Workaround

1. Make sure that the required user has not been deleted.

2. Follow Microsoft related documentation for the user checking, refer to:

 https://docs.microsoft.com/en-us/archive/blogs/spjr/sharepoint-troubleshooting-user-cannot-be-found-and-other-mysterious-user-behavior