Thanks doze, you are right, the placeholder is in the template.
@JuTs: Please make sure that the placeholder is in your template if you have modified the default galleryOuterTpl or if you created your own galleryOuterTpl.
And how can I upload images to my child galleries if I can see them ?
You can enter the URL of the gallery manually in order to upload images to it. You alternatively can use the preview feature of MODx. In the MODx Manager edit the page containing the childgallery and use the preview to access the "Manage Pictures" button. Once you have uploaded one image, the childgallery will show up in the list of childgalleries.
Maybe we should create a MODx module with wich galleries could be managed in the MODx Manager. For most of my customers it was quite irritating that they had to manage the galleries on the frontend of their sites. Beeing able to manage them in the MODx Manager would be a great improvement.