Atmega328p + V-USB Linux Problems

Hi,
I’m using the Arduino V-USB library (http://code.rancidbacon.com/ProjectLogArduinoUSB) on an Atmega328p to use it as a HID. This is working fine on Windows and Mac, on Linux (Lubuntu) however it’s not working properly – it gets disconnected and then reconnected all the time and I have no clue why. Do you have an idea what to change to make it work on Linux, too?
Here is the edited version of usbconfig.h http://pastebin.com/HbDGe3Pt.
Here is a my layout:


And a prototype of the layout:

Here is my basic code:

#include <HIDSerial.h>
#include "TimerOne.h"

HIDSerial serial;

void setup(){
  serial.begin();
  Timer1.initialize(1000);
  Timer1.attachInterrupt(poll);
}

// interrupt timer
void poll(){
    serial.poll();
}

void loop(){
  serial.print("a");
  delay(100); // arbitrary delay to limit data to serial port   
}

It would be so great if someone had an idea…
Thanks in advance,
Cheers,
Tim