Hi David!
Could you please check the response body if debug mode is enabled in wp-config.php? These are only the javascript error messages due to the server response.
Error 500 means, that at some operation the server stops the ajax process with an error 500. If debug mode is enabled, it might send the error message as well with the ajax response. You can check that on your browsers Network panel:
– before typing in anything: http://i.imgur.com/KBJnv4Z.png
– after typing in a phrase, you check the failed response: Step 1, Step 2
Hopefully there will be some kind of error message.