I didn't realize there was a screenshot in your first message. I'm unable to see the image, it says file broken like on the other discussion.
Can you maybe copy-paste the text of the error from the console?
If you enable debug in config.php, the console error might contain more information.
If it really is a server-side 500, it should be logged in Flarum log file. Or maybe it's a PHP general error that was logged to the system log file for PHP.