403 error when I try to update page

I built a site with Elementor and Astra theme a few weeks ago, which seemed fine. I’ve now gone back to the site to change a few things and am getting a 403 error message every time I try to update pages.
For some reason the home page can still be updated successfully, but all the other pages have the same problem. Exactly the same is happening on another site I manage.

-I’ve deactivated all my plugins and that doesn’t help

-I don’t currently have any security plugins apart from LoginLockdown, but I deactivated that as well.

-I’ve reverted to previous versions of Elementor

_I’ve reinstalled WordPress, I’m using v5.1

-I’ve added this to my WP config file:
define(‘WP_DEBUG’, true);
define(‘WP_DEBUG_LOG’, true);

-My hosting service tells me that they don’t have any firewalls that could cause problems and that the problem is with Elementor

-I’ve looked at the error logs but don’t understand them to be honest, here’s one from this morning:
[Mon Feb 25 13:17:05.247094 2019] [:error] [pid 28430:tid 140154303899392] [client 82.9.39.15:60704] [client 82.9.39.15] ModSecurity: [file “/etc/httpd/conf/modsecurity.d/rules/tortix/modsec/50_plesk_basic_asl_rules.conf”] [line “258”] [id “33350147”] [rev “143”] [msg “Protected by Atomicorp.com Basic Non-Realtime WAF Rules: Potentially Untrusted Web Content Detected”] [data “”] [severity “CRITICAL”] Access denied with code 403 (phase 2). Match of “rx ((?:submit(?:\\\\+| )?(request)?(?:\\\\+| )?>+|<<(?:\\\\+| )remove|(?:sign ?in|log ?(?:in|out)|next|modifier|envoyer|add|continue|weiter|account|results|select)?(?:\\\\+| )?>+)$|^< ?\\\\??(?: |\\\\+)?xml|^<samlp|^>> ?$)” against “ARGS:actions” required. [hostname “davidperris.co.uk”] [uri “/wp-admin/admin-ajax.php”] [unique_id “XHPq0Z1VQVhIwpw4hUgO7QAAANA”], referer: https://davidperris.co.uk/wp-admin/post.php?post=19&action=elementor

Can anyone help with this?
Thanks, Dave.

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