Server Error after updating a post (not with all the pages)

I am trying to add a ‘tag’ to a blog post we published before (without tags), but when we try to update the page, it says “Server error” we have checked our server (it is fine) and tried it in another post, and it worked. However, with that specific page, it doesn’t allow me to update anything of it. I have tested other pages and in some of them, I can publish the changes, but not in all of them.

Does anyone know why some pages can be updated and why other ones not?