When an assignment is created with the option 'Add due date to schedule' marked. A link to the assignment is added to the schedule tool. While the assignment is not open (not displayed in assignements tool yet), if a student goes to schedule and clicks on the link, it will get an Error on the UI.
The link should not be active until the assigment is open.
I believe that in olders versions of Sakai, an assignment was listed to students whether it was open or not. Now, an assignment is only visible to students if it is open, otherwise, they will not see the assignment.
CLE team thinks this was caused by the
SAK-11160 changes - 'Assignments which have not been released to students (as set by the dates in Assignments) are visible and clickable in the Calendar'