A 500 code is a general error that simply says the something has gone wrong on the webserver software and it could not be more specific.
Generally to troubleshoot these you will want to try clearing your browser cache and cookies and try again.
If that does not work (and I find it rarely does) then you will need dig a little deeper into the server side.
This means you will want to check your Apache access and error logs as well as your php error logs (if not combined) and see if there is anything logged there.
The most common causes are:
wrong file/directory permissions or ownership
a misconfigured .htaccess
note: many PHP production servers are set to not display errors in production environments, which generally means that the site issues a white page.
The error that you gave us is actually a warning and not an error, I don't think that it is the problem, but I will ask a dev to take a peak at this thread maybe they know something that I do not.