Hi, @Expiscornovus,
thanks for your help, I think there is not much left to solve this puzzle. 😄
I got another error, I think that is because I was not registered as an app owner. - changed that now.
Subsequently, I've triggered the workflow again unfortunately without success the error remains...

URL is now,
https://graph.microsoft.com/beta/users/6da38f53-ff1d-445b-8a06-262cc******/?$select=displayName,userPrincipalName,signInActivity
Error message,
{
"error": {
"code": "Authentication_MSGraphPermissionMissing",
"message": "Calling principal does not have required MSGraph permissions AuditLog.Read.All",
"innerError": {
"date": "2023-01-10T17:13:05",
"request-id": "bf8158c3-bee3-40d8-8941-f3456011a25d",
"client-request-id": "bf8158c3-bee3-40d8-8941-f3456011a25d"
}
}
}