Hey Cam,
That usually is a result of Mod Security blocking the X interface’s directory /x/#/. If you have access to disable Mod Security, you can quickly try if it’s causing the issue by disabling it. If not, please ask your web host to whitelist the said directory in Mod Security settings.
Another thing to try related to permalink issues is to reset your htaccess file by renaming it to .htaccess-bak. Then in WP Admin Menu, go to Settings > Permalinks and just click the Save Changes button.
And for good measure, contact your host to increase your allocated memory or do it yourself by adding this code in your wp-config.php
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
If none of the methods above works, please give us your site’s WordPress Login URL, username and password and also access to your web host’s cPanel or at least FTP.
Thanks.