roslaunch does not work but rusrun does

edit

Hi,

When starting my node "test_node" from the package "test_pck" with rosrun, everything works fine. Now i have written following launchfile named test.launch which i have placed in src/launch/

<launch>
<group ns="test">
  <node pkg="test_pck" name="test_name" type="test_node"/>
</group>
</launch>

however if i run "roslaunch test_pck test.launch" i gett following error in the terminal:

Traceback (most recent call last):
File "/opt/ros/indigo/bin/roslaunch", line 34, in <module>
import roslaunch
File "/opt/ros/indigo/lib/python2.7/dist-packages/roslaunch/__init__.py", line 61, in <module>
from .scriptapi import ROSLaunch
File "/opt/ros/indigo/lib/python2.7/dist-packages/roslaunch/scriptapi.py", line 42, in <module>
import roslaunch.parent
File "/opt/ros/indigo/lib/python2.7/dist-packages/roslaunch/parent.py", line 54, in <module>
import roslaunch.server
File "/opt/ros/indigo/lib/python2.7/dist-packages/roslaunch/server.py", line 78, in <module>
from rosgraph_msgs.msg import Log
File "/opt/ros/indigo/lib/python2.7/dist-packages/rosgraph_msgs/msg/__init__.py", line 2, in <module>
from ._Log import *
File "/opt/ros/indigo/lib/python2.7/dist-packages/rosgraph_msgs/msg/_Log.py", line 7, in <module>
import std_msgs.msg
ImportError: No module named msg

The terminal output of rospack find std_msgs is /opt/ros/indigo/share/std_msgs

I have absolutely now clue what i can do in order to solve this problem. Thank you for every hint!