@Mathias Klemt
Thank you for your post!
I wasn't able to replicate what you're seeing, but when it comes to checking what these Service Principals are, and to see if you're still actively using them, you can do this through the Azure Portal or PS.
Azure Portal:
1- Copy the Principal Name or Member Email (Application ID) into a notepad
2- Navigate to Azure Active Directory -> Enterprise Applications -> Select All Applications -> Search for the Principal Name (Application ID)
AzureAD PowerShell: Using the AzureAD modules for PowerShell
#Sign in to AzureAD
Connect-AzureAD
#You can find the ObjectID by following the Azure Portal Steps above
Get-AzureADServicePrincipal -ObjectId "<<Can be found using the AzurePortal steps above>>"
If you have any other questions, please let me know.
Thank you for your time and patience throughout this issue.
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