Error 502 Bad Gateway

Hello i found this error in my log, any one can tell me why its happen?
Also when object cache is on, i have the same problem with other plugins like “Autoptimize”.

[12-Jan-2018 19:45:31 UTC] PHP Notice: A feed could not be found at https://feeds.feedburner.com/W3TOTALCACHE. A feed with an invalid mime type may fall victim to this error, or SimplePie was unable to auto-discover it.. Use force_feed() if you are certain this URL is a real feed. in /sitename/wp-includes/class-simplepie.php on line 1555
[12-Jan-2018 19:45:31 UTC] PHP Notice: The data could not be converted to UTF-8. You MUST have either the iconv or mbstring extension installed. Upgrading to PHP 5.x (which includes iconv) is highly recommended. in /sitename/wp-includes/class-simplepie.php on line 1421