This problem can appear when using brand of On-Premises type and having website builder installed on your server. The problem can be caused by website builder plugin on Plesk having wrong website builder domain saved inside it and, therefore, when calling it — plugin throws error 404 not found. To fix the problem, update plugin on Plesk (installation instruction). If (1) does not help, check the IP address which your website builder domain is resolved to on your Plesk server. Try running the following command on Plesk server terminal: # curl http://[builder-domain]/api/status (replace "[builder-domain]" with a real domain of your website builder installation). If you do not get response {"ok":true}, it means that locally Plesk makes call to a different place, and not to website builder. In this case, check file "/etc/hosts" whether you have hardcoded IP for website builder domain specified. If yes, try to remove it (or temporarily comment) and check if website builder starts working.