Hi @B Mo ,
Thanks for reaching out and apologies for delay in response.
I understand you are using common endpoint to authenticate the user which should allow both Microsoft accounts and work or school accounts to access the application, but you are getting error while sign in with your Microsoft Account.
This can be due to sign-in audience need to be correctly configured in "supported account types" while registering the application.
Selecting the "Accounts in any organizational directory and personal Microsoft accounts" option will allow users who have personal Microsoft accounts and users from another tenant to access the application.
If you have registered your application initially as single tenant but wants to allow users from another tenant or Microsoft accounts as well. I would recommend to register the new application with above option to target largest sign-in audience to access your application.
Updating the account supported by an application will sometimes not allow to update application from single-tenant to multi-tenant due to Application ID URI (App ID URI) name collisions. So it is better to avoid that and register new application as multi-tenant application.
Reference docs: https://learn.microsoft.com/en-us/azure/active-directory/develop/quickstart-register-app#register-an-application
https://learn.microsoft.com/en-us/azure/active-directory/develop/howto-modify-supported-accounts#why-changing-to-multi-tenant-can-fail
Hope this will help.
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