The user I try to add is existing and activated. I have double checked that I use the correct Project and Grant ID; and the correct Org ID is passed as a header to the POST request. I have as well checked that I don't pass any role to the request that is not valid or not existing. Any help is more than welcome. Ref to API call details: https://zitadel.com/docs/apis/resources/mgmt/management-service-add-project-grant-member
Members are users with permission to administrate ZITADEL on different levels. This request adds a new user to the members list on the project grant level with one or multiple roles.