I am using a HTTP XML/REST Value sensor against a json feed. Every so often something in the json response will cause PRTG to display the error "Object reference not set to an instance of an object." instead of grabbing the requested value.
The feed can be found at http://blogs.mprnews.org/newscut/?json=1&include=id&count=1 I am grabbing the count_total value.
I ran the url through http://jsonlint.com/ and it says that the json is valid.