These internal errors from Google is a problem. You would expect "Internal Error" only to be given in case of some fatal error on Google's side. However, in many cases, I have found that they are due to the request being invalid in some way, and the Google programmers has just been lazy and not given a proper error message for a specific problem. Then the fall back error is just "Internal Error". This leaves us users of the API with little hint of what has gone wrong. I have raised this problem is several threads.

I suspect that in your case, it could be because you are using an MCC Account instead of a proper account, when you make the request. See the below link: http://groups.google.com/group/adwords-api/browse_thread/thread/4d3220feb56af0e/315df3c72c729ec1