2.1.5 Design Managmant - Default dont work

[ulli]

I have a new installed Website with version 2.1.5
and added a custom design.
- added a design - defined it as default design.
- added a template for core-site - associate with this design
- addes css and navigation - associate it to design
so far - all ok
then i wanted to set my core:site template as standard - it is not possible
when i create a new page , under options the design shown is not my defined default design (checked also via database that the correct design is set as default) which has the result that the page template is not correct..