The issue is not specific to wmf.9. It's not too serious - it goes away after reloading the page or navigating between pages.
Not too easy to reproduce.
- On testwiki log in as a user who has cross-wiki notifications (preferably from several wikis)
- After being logged in, click on the one of Notifications badges - the overlay will display 'Failed to fetch notifications'
Note: I put Request/Response headers into the comments