I would like to see the script and line where the error occurred instead of "500 - String could not be parsed as XML".
Is there any way to turn off joomla error reporting so I can just see standard PHP style errors.
I see two ways to do that, but you need access to server in ssh to perform (linux):
/var/log/apache2/error.log is a good start to know more about this error
usually i use and a command line tool like http://xmlsoft.org/xmllint.html that allows you to check for mal-formed xml files in whole file structure.
This is probably not the direct clue to your question, but maybe this can help. But this is how to solve (my way) a file / structure problem (upload, file missing, partials... etc)
OR, maybe simplier
as always in Joomla, you probablu use a third-party component, like someting connected to Google Map or other external webapp / api, and they changed their format and you are not aware of this... causing this sort of problem... so deactivate one by one and test... if you plan to do so I advice you to begin to deal with the plugin...