I believe you are running an older version of Panopticon. The message in the current version reads "The HTTP status code 500 means there has been a server error. Check if there are any third party plugins, especially system plugins, which may be accidentally breaking the Joomla! API Application and contact their author."
The correct way to deal with this is to go to your Joomla! site's administrator, set Site Debug to Yes, and Error Reporting to Maximum. Then visit the /api URL of your site. It will throw an error which will tell you which third party plugin is broken. Then you need to contact the developer of that plugin, show them the error trace you got, and show them this https://www.dionysopoulos.me/common-mistakes-writing-joomla-plugins.html If they refuse to fix it, please report the software to the Joomla! Extensions Directory as it's NOT compatible with Joomla! 4.0 or later, the developer refuses to fix it, therefore the software needs to be marked as non-compatible in the Joomla! Extensions Directory.
Nicholas K. Dionysopoulos
Lead Developer and Director
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