500 Internal Server Error means that there's something wrong with your server or your script, maybe high server load, memory usage, script settings problem, script error, etc.
You should check your error_log, web server error log, syslog, script settings, etc, or contact your hosting provider or plugin's author/support and ask whether they can help.
If the problem is temporary only, you can suppress the error by ignoring HTTP status code.
Need help? Reach out to support@fastcron.com - we're happy to help.
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