Fix Joomla 500 Error

My Joomla site suddenly isn’t pushing correct and throws a 500 error when a customer logs in, while inner pages still load when accessed directly. No extensions or plugins were recently installed or updated, but the build contains a few light template/code tweaks that must remain intact.

I need a quick, clean fix:
• Diagnose what is triggering the server-side 500 on the homepage (PHP, .htaccess, database, or mis-configured component).
• Apply the smallest effective change so the homepage and the rest of the site load normally again.
• Provide a short recap of what you found and what you altered, so I can track future changes.

Please be comfortable with Joomla core, custom overrides, PHP error logs, and typical shared-hosting environments.