Thanks for the update.
I’ve taken a backup of your site and tested it on my local PC — everything works fine with the same setup. The issue only occurs on your live site when trying to save, where the following URL shows a 404 error:
administrator/index.php?option=com_sppagebuilder&task=/editor.applicationSettings
This is usually caused by server-level security rules. I recommend checking with your hosting provider and asking them to review ModSecurity or firewall logs for blocked requests related to SP Page Builder. Temporarily disabling ModSecurity or whitelisting this request usually resolves the issue.
Please let me know the outcome.
Best regards,