Currently, every time I reopen Illustrator it forgets the configuration of the measurments. What can I do to change this ?
Did you also change the setting in the New Document-dialog?
The New Document dialog has the power to override your chosen units in Preferences, unfortunately.
The units in Preferences are only used when documents without units are created.
If you open an image or a PDF (saved without Illustrator editing capabilities), it will use the Preferences setting. Existing Illustrator documents or new documents (which are based on a preset) will keep their units.
Thank you for your help !
It was indeed the "new document" dialog.
In my opinion the "settings" dialog should have to power to overwrite this kind of documents ... (as it has been every time before ...)