The HTTP binding is interpreting the % as a start of a Java format specification. See…
Http Binding · openhab/openhab1-addons Wiki · GitHub (Dynamic URL Enhancement)
Use..
http://10.10.1.135/json.do?_PLN%%5B0%%5D
(double %) to workaround the binding formatting feature.
http://stackoverflow.com/questions/5011932/how-to-escape-in-string-format