I had this error when working locally on my laptop.
It took me a few minutes to figure out so I thought I'd blog this for future reference.
When loading an application running Fusebox 5.1 - I encountered the error in the title of this post.
The fix was pretty simple in the endup.
I simply had an invalid XML circuit file.
Fusebox normally tells you this using it's own errorHandling templates, but under certain conditions - this can happen.
If the error persists and your XML is valid - try setting the mode to production - especially in a production environment.