The error code 4xx and 5xx Can't resolve domain will be showing your domain name is not corrected with the current server IP.
What to do?
Error code 404 Can't resolve domain
1. Contact your Hosting Provider to check and fix your domain name to be corrected with Host name.
2. Access to RVsitebuilder to edit your website again.
Error code 403, 4xx, 5xx Can't resolve domain
1. You can check in cPanel -> File Manager
a) Owner group of public_html and all directories and files MUST be <user>:<user>
b) Permission of all directories under public_html MUST be 755.
c) Permission of all files under public_html MUST be 644
d) public_html MUST have file name "domainready.png"
2. If you cannot check these values in number 1, or found some incorrect values, please contact your Host Provider to check and fix it for you.
3. Once fixed, access to RVsitebuilder to edit your website again.