Try logging in step by step without resorting to using a bookmark, so by manually typing in the www-adress into the browser. You can only bookmark on the page before entering your username and password. If your link is very long, you are probably using an invalid bookmark.
If that does not work: Contact us at firstname.lastname@example.org and send us a screenshot of your error (comparable to the example picture above). Make sure the Unique Request ID is readable. Also tell us where exactly you are trying to log-in and what institution your account belongs to.
04-09-2019 - Do you need to go to "Mijn HSLeiden"? Google's toplink is broken, but you can just go to mijn.hleiden.nl directly