Post.php 404

WordPress 6.8.3 with Avada 7.13.3
Ultimate Member 2.10.6

I have a page called Client Area – it’s restricted so only logged-in users can see it.
(See the setup of the page here https://ibb.co/ynJ9cqrN )

When I edit the page and add another image for example – click update I get 404 from post.php

If I disable Ultimate Member, I can edit the page fine and no 404.

It did work perfectly and has suddenly broken. Any ideas what could be causing this and how to fix it?

Thank you!

The page I need help with: [log in to see the link]