You are smart to try deactivating plugins, though when you say “all plugins deactivated and active” it’s self contradictory. They can’t be both at the same time ?? I assume you meant deactivated but still installed, which is fine.
Themes can also cause aberrant behavior. You should test with all plugin deactivated and using one of the default Twenty* themes.
When you add a page to a menu, WP makes an Ajax call to itself to register the change. This is failing for some unknown reason. Immediately after such a failure, check your error log for any related messages. Even a non-fatal, minor “Notice” level message could indicate why the Ajax call is failing.
Ajax failure can be a result of improper security measures. For example, the server’s modSecurity configuration might need adjustment.
It could be your core installation had become corrupted. You many need to perform a manual “update”, even if it’s to the same version. The point is to manually refresh all the files from a fresh download. Please do so manually, the “Re-install version 6.3.1” button on the updates admin screen is less than 100% reliable for this.
