Xenforo - Web Server Optimisation

My site has been recently upgraded to v2.0.4 - had been running on 1.x version without any problems for a long time. File Health is fine. Upgrade performed by Xenforo expert. Unfortunately - since the upgrade it's been throwing up internal 500 server errors. Host is a bit clueless on how to resolve these issues, so I'm looking for someone who's good at looking at logs and figuring out how to resolve these issues would be preferred. I've included the reply from my host in this message. (domain name has been replaced by XXXdomain-nameXXX) for privacy. Cpanel/WHM/FTP access can all be provided. I need to know if you're able to resolve it.