To activate WP_DEBUG, you’ll need to access your FTP. You may ask assistance from your hosting provider with regards to this. Here’s an article that might help as well: https://www.pipeten.com/support/applications/enabling-wp-debug-in-wordpress/
Can you also right click and inspect while editing a page, then go to console, and click update? Check if any error will appear upon updating. Example: https://share.getcloudapp.com/ApuXLxkv
Can you also check if you have CDN or server side(ask your hosting provider as well with regards to this) cache? Kindly clear them if so.
Lastly, since the issue exists even when using different themes, perhaps you can reach out to your hosting provider as well if they have an insight with regards to this?