Hi @Frits ,
I believe I answered your question in this post. Re-posting here for visibility:
"The App Service team investigated this issue and it turns out it can be resolved by adding the account configuring the App Service into a Global Administrator role in AAD. A Global Administrator can manage all aspects of Azure AD and Microsoft services that use Azure AD identities. There can be more than one Global Administrator at your company. Global Administrators can reset the password for any user and all other administrators.
If you are not the owner of the subscription, the owner can add you as a Global Admin by following the steps in this documentation: Assign administrator and non-administrator roles to users with Azure Active Directory. "
If you have further questions please let me know.
Thanks,
Grace