This fixed it for me:
Modsecurity config file had some bad code. I set that back to default and everything starts.
As admin, go to Security>Mod Security
Click "Edit Rules" next to each one, and hit the "Edit File" link at the bottom.
Shows you the contents of:
"/usr/local/apache/conf/userdata/USERNAME/example.com/modsec.conf"
Mine was basically corrupted. I set back to:
<IfModule mod_security2.c>
SecRuleEngine On
</IfModule>
If your file is fine, try checking the other related security files. If you ever tried to manually edit any of those, it is a good chance thats the problem. Could just be a missing > or duplicate code.
Hope it helps someone else!