Hi there,
The issue may be related to ModSecurity. Please disable it from your hosting provider, and the problem might be resolved.
I have tested the site in a local environment, and everything is working perfectly fine, including drag-and-drop functionality, saving sections, and saving new items.
Please check the screenshot in the hidden content section:
Additionally, your site may have firewall rules that are blocking the IP multiple times. I also tried accessing it several times using a VPN.
Screenshot
Feel free to ask if you have any further questions.
Best regards