The message "Could not update project preference" is displayed in the Clarity PPM Modern UX when you either
open any project with any user, click on the project detail view then a) click F5 or b) click the URL and then hit "Enter"
This does not happen with any of the other project views in the Modern UX
This is caused by defect DE51276.
Release : 15.7
Component : CA PPM PROJECT MANAGEMENT
Fixed in Clarity PPM 15.8