An AJAX HTTP error occurred

I installed the module, and made a new view as described in the readme.txt file.

I got to selecting the field I wanted to filter using vsf and when I tried to add filter criteria this popped up:

An AJAX HTTP error occurred.
HTTP Result Code: 500
Debugging information follows.
Path: http://www.mysite.com/admin/structure/views/ajax/add-item/test/page/filter
StatusText: Internal Server Error
ResponseText:

I closed this pop up box and tried to do it again but got the same message.

Any ideas, suggestions?

Thanks