Thanks for your reply.
Indeed, I didnt install imagej yet. After installing imagej, I am still getting the same error.
I thought about re-installing cellprofiler, but uninstalling also errors:
Can't uninstall 'CellProfiler'. No files were found to uninstall.
Maybe the installation did not fully complete (although there was no error message after the installation)?
Moreover, running the pip command as in the wiki gives following message:
The directory '/home/jens/.cache/pip/http' or its parent directory is not owned by the current user and the cache has been disabled. Please check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.
Therefore, i run pip with sudo -H. I dont know if that has anything to do with the problem here.