Here is my Fhem config.

attr global userattr cmdIcon devStateIcon devStateStyle icon sortby webCmd widgetOverride
attr global autoload_undefined_devices 1
attr global logfile ./log/fhem-%Y-%m.log
attr global modpath .
attr global motd SecurityCheck:\
  \
WEB,WEBphone,WEBtablet has no associated allowed device with basicAuth.\
telnetPort has no associated allowed device with password/globalpassword.\
\
Restart FHEM for a new check if the problem is fixed,\
or set the global attribute motd to none to supress this message.\

attr global statefile ./log/fhem.save
attr global updateInBackground 1
attr global verbose 3

define telnetPort telnet 7072 global

define WEB FHEMWEB 8083 global

define WEBphone FHEMWEB 8084 global
attr WEBphone stylesheetPrefix smallscreen

define WEBtablet FHEMWEB 8085 global
attr WEBtablet stylesheetPrefix touchpad

# Fake FileLog entry, to access the fhem log from FHEMWEB 
define Logfile FileLog ./log/fhem-%Y-%m.log fakelog

define autocreate autocreate
attr autocreate filelog ./log/%NAME-%Y.log

define eventTypes eventTypes ./log/eventTypes.txt

# Disable this to avoid looking for new USB devices on startup
define initialUsbCheck notify global:INITIALIZED usb create
define gateway MYSENSORS /dev/ttyUSB0@57600
attr gateway autocreate 1
attr gateway first-sensorid 10
attr gateway icon it_wireless_dcf77
attr gateway stateFormat connection
define UVSensor MYSENSORS_DEVICE 10
attr UVSensor IODev gateway
attr UVSensor mapReading_uv 0 uv
attr UVSensor mode node
attr UVSensor version 1.2

And a standard UV mysensor Arduino code (no mods).

/**
 * The MySensors Arduino library handles the wireless radio link and protocol
 * between your home built sensors/actuators and HA controller of choice.
 * The sensors forms a self healing radio network with optional repeaters. Each
 * repeater and gateway builds a routing tables in EEPROM which keeps track of the
 * network topology allowing messages to be routed to nodes.
 *
 * Created by Henrik Ekblad <henrik.ekblad@mysensors.org>
 * Copyright (C) 2013-2015 Sensnology AB
 * Full contributor list: https://github.com/mysensors/Arduino/graphs/contributors
 *
 * Documentation: http://www.mysensors.org
 * Support Forum: http://forum.mysensors.org
 *
* This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * version 2 as published by the Free Software Foundation.
 *
 *******************************
 *
 * REVISION HISTORY
 * Version 1.0 - epierre
 * Contribution: bulldoglowell, gizmocuz
 * 
 * DESCRIPTION
 * Arduino UVM-30A
 * Index table taken from: http://www.elecrow.com/sensors-c-111/environment-c-111_112            
/uv-sensor-moduleuvm30a-p-716.html
 * Because this table is pretty lineair, we can calculate a UVI with one decimal 
 *
 * Connect sensor:
 *
 *   +   >>> 5V
 *   -   >>> GND
 *   out >>> A0     
 * 
 * License: Attribution-NonCommercial-ShareAlike 3.0 Unported (CC BY-NC-SA 3.0)
 */

#include <SPI.h>
#include <MySensor.h>  
#include <MySensor.h>  
#include <SPI.h>

#define UV_SENSOR_ANALOG_PIN 0

#define CHILD_ID_UV 0

unsigned long SLEEP_TIME = 30*1000; // Sleep time between reads (in milliseconds)

MySensor gw;
MyMessage uvMsg(CHILD_ID_UV, V_UV);

unsigned long lastSend =0; 
float uvIndex;
float lastUV = -1;
uint16_t uvIndexValue [12] = { 50, 227, 318, 408, 503, 606, 696, 795, 881, 976, 1079, 1170};

void setup()  
{ 
  gw.begin();

  // Send the sketch version information to the gateway and Controller
  gw.sendSketchInfo("UV Sensor", "1.2");

  // Register all sensors to gateway (they will be created as child devices)
  gw.present(CHILD_ID_UV, S_UV);
}

void loop()      
{
  unsigned long currentTime = millis();

  uint16_t uv = analogRead(UV_SENSOR_ANALOG_PIN);// Get UV value
  if (uv>1170)
    uv=1170;

  //Serial.print("UV Analog reading: ");
  //Serial.println(uv);

  int i;
  for (i = 0; i < 12; i++)
  {
    if (uv <= uvIndexValue[i]) 
    {
      uvIndex = i;
      break;
    }
  }

  //calculate 1 decimal if possible
  if (i>0) {
    float vRange=uvIndexValue[i]-uvIndexValue[i-1];
    float vCalc=uv-uvIndexValue[i-1];
    uvIndex+=(1.0/vRange)*vCalc-1.0;
  }

  //Serial.print("UVI: ");
  //Serial.println(uvIndex,2);

  //Send value to gateway if changed, or at least every 5 minutes
  if ((uvIndex != lastUV)||(currentTime-lastSend >= 5UL*60UL*1000UL)) {
      lastSend=currentTime;
      gw.send(uvMsg.set(uvIndex,2));
      lastUV = uvIndex;
  }

  gw.sleep(SLEEP_TIME);

}

Use MySensors v1.4.2 in your gateway? I only use Ethernet (W5100) gateway so I can't test, but MySensors v1.5 and later isn't supported by Fhem yet AFAIK.

Tried static Node IDs?

Post your fhem.cfg, or at least the relevant parts.