This release group http://musicbrainz.org/release-group/326041d8-be37-448f-8067-db6c110292ca had a release in it, but it has been deleted. I haven't found where to delete the relase griup as well. Can anyone help?
AFAIK empty release groups will be deleted automatically, just like e.g. empty artists.
Release group will get automatically deleted when there's no releases and it doesn't have any relationships. On this case group wasn't removed because of relationship. I made an edit to remove it: http://musicbrainz.org/edit/38848620