I suggest you temporarily use the default codec for your input, comment out all filters and outputs, and add a stdout { codec => rubydebug } output to dump the raw data that you receive. You're obviously not receiving valid JSON so first we need to figure out why.
I'm not convinced that the chosen way of getting rsyslog to send JSON is a great idea. What happens if the message contains a double quote? I don't see anything in that configuration snippet that makes sure that it gets escaped.