Hi @minke , I'll repost my answer from the linked thread above for reference.
The error message you're receiving is because you're using the /common endpoint, you should be using a tenant specific endpoint.
Error message:
Usage of the /common endpoint is not supported for such applications created after '10/15/2018'. Use a tenant-specific endpoint or configure the application to be multi-tenant.
Endpoint example of /common - https://login.microsoftonline.com/common
You should be using a tenant specific - https://login.microsoftonline.com/contoso.onmicrosoft.com
Please let me know if this fixes your problem or if you have any questions.
If this answer helped you please mark it as "Verified" so other users may reference it.
Thank you,
James