Notification filter preference cleanup events cannot be sent to the other cluster members

Proposed new title, to name the symptom rather than the mechanism (the fix keeps this event on the member performing the cleanup, so the current title would read in the release notes as if the events could now be sent): Deleting a page watched by users logs serialization errors when clustering is enabled. The account used to create this issue cannot edit its fields; could someone with the rights rename it? The commit and the pull request will then use the new title.