Grails 3 and Json: "Request method 'POST' not supported"

I'm porting a working webapp from Grails 2.3 to 3.0.1. When I post the Json string {"command":"ping"} to the server i get the following result:

{"timestamp":1429380750958,"status":405,"error":"Method Not Allowed","exception":"org.springframework.web.HttpRequestMethodNotSupportedException","message":"Request method 'POST' not supported","path":"/rps/request"}

Here's the controller:

import org.grails.web.json.JSONObject
class RequestController {
    def index() {
        def jsonManagerService
        JSONObject json = request.JSON
        if(!json){
            render "{json or gtfo}"
            return
        }
        render jsonManagerService.parseJson(json)
    }
}

And here's the JsonManagerService:

import grails.transaction.Transactional
import org.grails.web.json.JSONObject
@Transactional
class JsonManagerService {
    def parseJson(JSONObject o) {
        switch  (o.command){
            case("ping"):
                return '{"result":"pong"}'
                break;
            default:
                return '{"result":"unknown command"}'
        }
    }
}

It looks like a Spring related issue. All the search on this matter provided no results. Any idea?