Strangely, I have from time to time an error when creating a landing page.
I try to create a page, I have an error, and then a 404 and I can't do anything about it.
Strangely, I can't reproduce steps that make this happen or not, but it happened at list 10 times during my tests.

See the screenshot.

I can't provide more input than that.
If I have new test case where it happens, I'll let you know.