DUK

The problem should be solved with plugin re-installation.

This is because PHP is currently working as CGI program on virtual host. To fix this log in to Plesk as admin. Edit service plan which is used by current website. Select tab "Hosting Parameters". Change "Run PHP as" to "Apache module".

Site.pro provides plugins from Plesk 12 (and higher) till Plesk Onyx.

To solve this problem please re-install the plugin on your Plesk server downloading new plugin from your license which you use on Site.pro. It will return back your icon and title and the plugin overwriting should never happen again.
Please note that this action is absolutely safe — it will not affect clients websites.

Builder requires FTP passive mode enabled on your server to be able to publish websites. Please refer this topic to enable FTP passive mode on Plesk:
https://docs.plesk.com/en-US/obsidian/administrator-guide/server-administration/plesk-for-linux-configuring-passive-ftp-mode.74643/.

This problem can appear when using brand of On-Premises type and having builder installed on your server.

1. The problem can be caused by builder plugin on Plesk having wrong builder domain saved inside it and therefore when calling it — plugin throws error 404 not found. It can occur when you have plugin installed before builder installation (and correct builder domain assigning) in the brand. In this case the plugin re-installation on Plesk can fix the problem. Open your brand and click button "Install plugin" in brand section "Install plugin" to open installation/update guide.

2. If (1) does not help then check the IP address which your builder domain is resolved to on your Plesk server. Try running the following command on Plesk server terminal:
# curl http://[builder-domain]/api/status
(where replace "[builder-domain]" with a real domain of your builder installation).
If you do not get response {"ok":true} then it means that locally Plesk makes call to a different place, and not to builder. In this case check file "/etc/hosts" whether you have hardcoded IP for builder domain specified. If yes, try to remove it (or temporarily comment) and check if builder starts working.