As soon as I saw the headline, I knew this was due to a WAF.
I worked on a project where we had to use a WAF for compliance reasons. It was a game of wack-a-mole to fix all the places where standard rules broke the application or blocked legitimate requests.
One notable, and related example is any request with the string "../" was blocked, because it might be a path traversal attack. Of course, it is more common that someone just put a relative path in their document.