I checked with my hosting company and this was their response:
On checking the error logs I could see that ModSecurity which is a Plesk Web application firewall is blocking the WordPress plugin due to which you’re facing the issue…Please note that Mod_security is an Apache module that helps to protect your website from various attacks.
They have temporarily turned off ModSecurity with.htaccess. Obviously, something is not right with the Gutenberg beta code or my hosting company settings.
What do you think?
Charles