Hi @Phillip Senger (senger@senger-prager.de)
is this user being synced from an on-premises Active Directory or is this a cloud-only identity?
Did you only change the users display name and not their UPN (or sign-in name)?
The blog post here explains the process of changing the UPN for the user:
https://ourcloudnetwork.com/how-to-change-a-user-upn-in-office-365-with-powershell/
Thanks
Michael Durkan
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