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.