603 Error ‘Access Denied’ - Authentication is successful but user doesn’t have sufficient permission to call this API. Additional permissions may need to be assigned to the user role.
A 603 Marketo error is thrown in the Marketo Admin Notifications if users attempt to make API calls to endpoints that they have not enabled Marketo API permissions for, and if these errors are experienced then the Marketo account setup needs to be reviewed. It is important to note that if these errors are only seen for endpoints such as /companies/describe during authentication and access token refresh then the cause of the 603 error may be due to the API calls that are made to build metadata when calling the Marketo /describe endpoints. If that is the case in your scenario then these 603 errors will not inherently cause issues for your users, but the Marketo account and permissions should still be reviewed.
Note that a 603 error is not passed through to Cloud Elements, and is only visible through the Marketo Admin Notifications. If you need to find out which object was accessed that caused this 603 error with certainty you will need to reach out to Marketo directly for assistance.