If you learn the source of this error, or how to fix it, let me know. I can't find where it's coming from.
You can turn off the log_deprecated System Setting to stop that error from being entered in the log, but I'm guessing it's unrelated to your 500 error.
PHP 7 (which I suspect you have) is much more likely than earlier versions to throw a 500 error when it encounters a PHP issue.
It could be an outdated plugin or snippet.
Sometimes, watching the Network tab in Chrome Dev. Tools (Ctrl-shift-i or F12) while loading a page will give a clue. If there's a connector.php call, you can click on it and look at the response tab, which might contain a PHP error message (buried in some dense HTML).
As Bob says, that error in your log is unrelated to a 500 error. The deprecated message is logged by mistake and has been corrected for 2.7.1: https://github.com/modxcms/revolution/pull/14162