calibre will not start

Hallo,

new problem for me:
an a new installed windows 10, calibre won't start after installing.
I tried:
- latest 64 bit
- latest 32 bit
- portable version
- windows 8 compatible version

all with the same behavior; will start to the taskmanager, where "the main calibre program" will show up, but nothing else.

Any ideas where to look at?

Thank you
Arne