Could not parse response into a JSON object

I think my problem is also looks similar to this one

Getting following error when system is under load

jsonPath($.user.id).find.exists failed, could not prepare: Boon failed to parse into a valid AST: JsonException: Unable to determine the current character, it is not a string, number, array, or object The current character read is ‘<’ with an int value of 60 Unable to determine the current character, it is not a string, number, array, or object line number 16 index number 15 ^

jsonPath($.access_token).find.noop failed, could not prepare: Boon failed to parse into a valid AST: JsonException: Unable to determine the current character, it is not a string, number, array, or object The current character read is ‘T’ with an int value of 84 Unable to determine the current character, it is not a string, number, array, or object line number 1 index number 0 The specified CGI application encountered an error and the server terminated the process. ^

Any Idea ?