In case if ISPmanager is running under a different port than default 1500 - check if the proxy is switched on for images. Apparently, the address of the 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 (for example: "https://domain.com/logo.png the" -> "https://domain.com:1500/logo.png"). If the picture opens, in this case the problem is as described above.