I have the same problem. It’s caused me some heartache.
Wordpress site states in requirements PHP 5.6 or greater so not supporting PHP 7 is a serious issue.
It works fine with PHP 5.6.
I have PHP XML installed and it still fails like this:
[Sat Oct 08 17:14:20.132351 2016] [:error] [pid 11294] [client 172.31.1.122:3871] PHP Fatal error: Uncaught Error: Call to undefined function simplexml_load_string() in /home/ubuntu/theweedblog/public/wp-content/plugins/wp-ses/ses.class.0.8.6.php:635\nStack trace:\n#0 /home/ubuntu/theweedblog/public/wp-content/plugins/wp-se$