This question has been answered by BobRay. See the first response.
That usually means that some snippet on the page or template (or in a plugin) contains a syntax error or other problem. PHP 7 (I assume that's what you have) is much less forgiving than earlier versions and will throw a very unhelpful 500 error at the drop of a hat.
It's worth checking the MODX error log: Manage->Reports->Error Log