Hi Tully,

> Could you check some of the debugging outputs from tf.  There are debugging
> methods here:  http://www.ros.org/wiki/tf/Debugging%20tools  Of particular
> interest is the output of view_frames.

I did check the output of view_frames and there is only ONE tree, also  
the TF-display of rviz display the tree correctly.

> Also, are you running in simulation?  If so do you have /using_sim_time set
> to true?  This can cause a race condition at startup and cause the "no
> common time" error.  See documentation here http://www.ros.org/wiki/Clock

No, I am not running in simulation but on the real robot (a P3AT).

The most strange thing is - in my opinion - that I can get the correct  
transformation with tf_echo while my code is throwing this error.

Regards,
Sabrina