Why can I not login with my alumnus account?
As graduate (alumnus) of Leiden University, you can not use your (former) s-number to log into Brightspace. Only students with a current registration at Leiden University have access to Brightspace.
If you want to follow a course at the University as a graduate, you can contact the administration of the service and/or faculty. They will be able to grant you access to Brightspace with a new guest-account: you will receive new log-in name. They will also make sure you will be enrolled in the course in Brightspace.