Hello @ViolainePrior
I am sorry you are having trouble assigning different dates for each of your students.
I know with the recent update, the process for assigning different due dates to students has changed the UI a little bit in the last couple weeks.
Just to rule out some simple things, are the due dates you are applying to the assignment falling within the course Start and End dates?
After selecting the Apply and Save, are you seeing any odd error messages?
Does clearing the cache and cookies (MAC) (PC) make any difference at all?
Do you see this same behavior while using a different browser?
-Colton