Backend saving returns ‘Undefined’ + layout lock issue

Hey Simm,
May I bother you again?

For a few days now, I keep getting “Undefined” as a response whenever I save something in the backend. The same problem occurs in the layout when I click on Lock. See a few images for illustration.

Could this be related to WP version 6.9.4 that was installed a few days ago?
My main concern right now is that bookings for my event are open, and I’m afraid that information might get lost. Updating the event in the layout is also not running smoothly.

https://pcdenegger.be/wp-content/uploads/2026/03/Undefined2.jpg
https://pcdenegger.be/wp-content/uploads/2026/03/Undefined.jpg