Sure, here are some common auth-related errors in .NET Core API:

• Unauthorized (401): This error occurs when the user is not authenticated. This can happen if the user does not provide a valid username and password, or if the user's credentials have expired.
• Forbidden (403): This error occurs when the user is authenticated but does not have the necessary permissions to access the resource. This can happen if the user is not a member of the correct role, or if the user does not have the correct permission level.
• Invalid grant (400): This error occurs when the authentication token is invalid. This can happen if the token is expired, if the token has been tampered with, or if the token is not from a trusted source.
• Invalid client (400): This error occurs when the client is not authorized to access the resource. This can happen if the client is not registered with the authorization server, or if the client's credentials have expired.
• Server error (500): This error occurs when there is an internal error in the authentication server. This can happen due to a variety of reasons, such as a database error or a network error.

To troubleshoot these errors, you can use the following steps:

1. Check the authentication configuration. Make sure that the username and password are correct, and that the user has the necessary permissions to access the resource.
2. Check the authentication token. Make sure that the token is valid and has not expired.
3. Check the client configuration. Make sure that the client is registered with the authorization server, and that the client's credentials have not expired.
4. Check the server logs. The server logs may contain more information about the error.

If you are still unable to troubleshoot the error, you can contact the support team for your authentication server.