Steps to reproduce

1. Login as Admin
2. Setup the Mail Sending on the instance
3. Create an user (e.g. U1) with a valid email address
4. Login as the user
5. Click on the Notifications Bell and turn ON all Watch Notification Toggles
6. Login as Admin
7. Create a page (e.g. 'Pag1') and save it
8. Edit the page again, add some content and save it
9. Comment on the page
10. Go to <server>/xwiki/bin/view/Scheduler/ and click on "Trigger" button for the 'Notifications daily email' job
11. Observe the notification events in the email received by the user

Actual results

The notifications events from the mail received by the user have a proper description (e.g. 'Pag1 commented by Administrator').

Expected results

Regardless of the type of event, the description is a string:

#displayCompositeEventDescription($event)

The issue could not be reproduced on XWiki 15.7, 15.5.3 or 14.10.19.