Spanning backup do not support backup for Guest Microsoft Entra(Active Directory) users as Guest users don't have a license for your Microsoft 365 tenant and Spanning cannot backup users without license.
Why did I see guest users previously?
Before July 2024 Spanning backup showed all users in the domain.
How to check if user has a guest type?
In the Users list in Microsoft Entra, you can use Add filter to display only the guest users in your directory
