Stopped by the Component Dispatcher - something deeper is going on, perphaps even at the server level. Sometimes I've run into problems with firewalls and mod security settings. My assumption is you have already re-installed Joomla core files, Helix, and SPPB. If this was built from a quickstart, some server assumptions may be wired in that might be corrected in a backup and reinstallation. Is there an update for the theme? Do you have access to a different server where you could install an Akeeba backup?
If you open the parent domain and view the page source you will see quite a few errors listed, incipit "COM_SPPAGEBUILDER". This is also a component and something Joomshaper might need to seriously look at, not just "apologize for the inconvenience."
One error I see relates to WebAuthn - you should go to plugins and turn this off. Also, since there are quite a few permissioning problems, try creating an alternate superuser account for yourself.
to view source, extend this by putting your domain name right after view-source:https://