Hello Sir,
I can fix it in 10 minutes if we can have a chat.
I have checked your requirements about your add/edit post/page error.
I have been fond of this error, it might be due to a Plugin incompatibility.
I am glad to say I am capable of doing this job because I have good experience with WordPress bug fixing, HTML, CSS, JS, PHP, Bootstrap.
I need a bit of explanation though on which version of WordPress do you have?
Also can i know the Website?
Please message me so that we can discuss so that I can show you my Works if you need them.
Thanks,
Nimesh