Solved:
The problem was that the module translated the server name into an up address and CherryPy was not configured to response to that.
Reconfiguring did the trick:
this article explains how this is done
Thanks for your efforts.

@cowboysdude I added the header (without the this.config.email) but got the same result.

@E3V3A Any idea what I could do to accept the connection?

@ninjabreadman Yes, curl call returns correct json:

{"Date": "22.02.2018", "Temperature": -0.187, "Time": "22:45:37"}

Hi all,
I like to display a simple value that can be fetched from a JSON service. So I started with the MMM-json-feed module which is pretty much doing the trick with some test services that I found (e.g. http://ip.jsontest.com/).
It seemed to need just a simple adaptation of the config file, to load the JSON data from an URL in my private network.
Unfortunately this does not work, although my JSON service returns proper data (checked with my browser). The mm output is

MMM-json-feed sending request: GET http://raspberrypi.fritz.box/temp/
MMM-json-feed received
MMM-json-feed Error: connect ECONNREFUSED 192.168.178.33:80
Whoops! There was an uncaught exception...
TypeError: Cannot read property 'statusCode' of undefined
    at Request._callback (/home/pi/MagicMirror/modules/MMM-json-feed/node_helper.js:15:48)
    at self.callback (/home/pi/MagicMirror/node_modules/request/request.js:186:22)
    at emitOne (events.js:96:13)
    at Request.emit (events.js:191:7)
    at Request.onRequestError (/home/pi/MagicMirror/node_modules/request/request.js:878:8)
    at emitOne (events.js:96:13)
    at ClientRequest.emit (events.js:191:7)
    at Socket.socketErrorListener (_http_client.js:358:9)
    at emitOne (events.js:96:13)
    at Socket.emit (events.js:191:7)
MagicMirror will not quit, but it might be a good idea to check why this happened. Maybe no internet connection?
If you think this really is an issue, please open an issue on GitHub: https://github.com/MichMich/MagicMirror/issues

I thought it might relate to CORS and compared the headers of the JSONTest (http://ip.jsontest.com) and my JSON service. They both look quite similar. Nothing that make me believe there is something missing.
JSONTest:

Access-Control-Allow-Origin: *
Cache-Control: private
Content-Encoding: gzip
Content-Length: 67
Content-Type: application/json; charset=ISO-8859-1
Date: Wed, 21 Feb 2018 21:08:30 GMT
Server: Google Frontend
Vary: Accept-Encoding
X-Cloud-Trace-Context: f1d4981eae5655f530221b705a794f51

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding: gzip, deflate
Accept-Language: de,en-US;q=0.7,en;q=0.3
Connection: keep-alive
Host: ip.jsontest.com
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:58.0) Gecko/20100101 Firefox/58.0

My service:

Access-Control-Allow-Origin: *
Content-Length: 64
Content-Type: application/json
Date: Wed, 21 Feb 2018 21:08:06 GMT
Server: CherryPy/13.1.0

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding: gzip, deflate
Accept-Language: de,en-US;q=0.7,en;q=0.3
Cache-Control: max-age=0
Connection: keep-alive
Host: raspberrypi.fritz.box
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:58.0) Gecko/20100101 Firefox/58.0

Any guesses?