ISPmanager

Make sure that PHP extension "curl" is turned on. Also make sure that PHP parameter "disable_functions" doesn't include function "shell_exec". If it is included, delete it from there and restart server.

Site.pro provides plugin for ispmanager (version 5 and higher). Site.pro does not provide/support plugin for ispmanager 4.

To fix this issue try deleting those FTP account(s) of that particular user that have username of format "s[8-digit-hash]" (e.g., "s1a2b3c4d") on your ispmanager account. After that open website builder from ispmanager again and try to publish website.

In case ispmanager is running under a different port than default 1500, check if the proxy is switched on for images. Apparently, if the address of website builder icon is without port 1500, so the image cannot be found.
If the logo of the website builder does not appear, it is necessary to check its source in the developer tools (URL). If it is without any port, try to add port to the URL (e.g., "https://domain.com/logo.png the" -> "https://domain.com:1500/logo.png"). If the picture opens, the problem is as described above.