Hi Paul,
First thing I'd do is create a new page, drop a qListForm on it and configure it exactly the same as the one that gives you the error.
If that works then delete the faulty one and recreate it. If th error persists then go to the ULS log file and try to find the log entry for the error. You can use a tool (http://archive.msdn.microsoft.com/ULSViewer this is from Microsoft) to make the search easier.
Another thing that you may consider would be upgrading to the latest version of QWP 5.7.
Please let me know how it goes.
Thanks,