I just downloaded Grails 3.0.0 (hoping to see my problems with CAS magically disappearing ;) ) I installed it under windows and then:
D:\\GrailsProjects
ok that was trivial (and rather stupid). In the "project directory" among other projects there was a directory called grails-app, probably leftover of some porting. This caused the create-app to fail. Removed the directory everything works fine.