The exact cause of a 404 error can vary, but if you're receiving it when updating Elementor or making any other kind of call to
admin-ajax.php and it only happens when performing very specific actions, then there's a good chance that it's security related.
I've had this error happen several times across several completely different WordPress sites. It doesn't seem to be specifically related to Elementor, though in my experience it happens more frequently with Elementor.
In every case that I've had so far, the cause was that the network request was being blocked by a security rule (false positive).
One time it was due to WordFence, so I used their Learning Mode feature to whitelist the action.
Another time it was triggered by Namecheap's mod_security rules, so I contacted their support and had them whitelist it.