image_proc nodelet with launch file is neither subscribing nor publishing

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Hi, I´m am currently running ROS melodic 1.14.5 on Ubuntu 18.04.4 and working on an stereo-vision project. I want to run two image_proc/rectify nodelets from the image_pipeline-stack, to rectify the raw images of my cameras and a self -written nodelet for the correspondence-analysis with a single nodelet manager. For this purpose i´ve already written a launch file:

<launch>
<arg name="manager" default="stereo_manager"/>

<!-- launching the nodelet manager, which manages all nodelets-->
<node pkg="nodelet" type="nodelet" name="$(arg manager)" args="manager" output="screen" />

<!-- launching the rectification nodelet for the left camera in the namespace of the left camera-->
<group ns="/wide_stereo/left">
<node pkg="nodelet" type="nodelet" name="rectification_left" args="load image_proc/rectify $(arg manager)" output="screen"/>
</group>

<!-- launching the rectification nodelet for the right camera in the namespace of the right camera-->
<group ns="/wide_stereo/right">
<node pkg="nodelet" type="nodelet" name="rectification_right" args="load image_proc/rectify $(arg manager)" output="screen"/>
</group>

<!-- launching my own stereo_vision nodelet which subscribes to the /image_rect - topics of both cameras-->
<node pkg="nodelet" type="nodelet" name="stereo_vision" args="load stereo_vision/StereoVision $(arg manager)" output="screen"/>
</launch>

This successfully launches all the nodelets, but both rectification-nodelets neither subscribe to the topics of the corresponding cameras, nor publish a /image_rect topic, as they are supposed to.

Interestingly image_proc works, when i am starting it with:

ROS_NAMESPACE=/wide_stereo/"either left or right" rosrun image_proc image_proc

Can anyone help me?

Thanks in advance

Tim