The most common reasons are:
Your server may have set too sensitive security level set, or
PHP file_get_contents() function has been disabled/locked
- in that case, please contact hosting support.
** In most cases can be changed in php.ini or .htaccess file or ask Hosting support.
Check also below settings:
Turn on the allow_url_fopen from your hosting
Check the console error using inspect element from your browser
Rocket loader or mod_security may block the resource loading.
That's why: Turn off the Rocket loader, if you use the SSL certificate.
Rocket loader contains inside the Cloudflare SSL Certificate.
Turn off (disable) mod_security from cPanel (hosting settings).
Use FTP tool (or File Manager) and check if the com_sppagebuilder folder exists or not. If exist delete (or rename) the folder: templates/your_template/html/com_sppagebuilder