photos not streamed / uploaded

I have tried this in iPhoto and Aperture, but my photo stream is not uploaded. All the photos are visible in WEB/Photostream, but they don't appear on my iOS devices.

And when I want to make these photos public in Aperture, to make them visible for anyone, the √ vanishes a second after I hit the checkbox.

Flickr stream pix do not show up either although the account looks OK.

Looks as if something was blocking Aperture's traffic, but I have nothing of that kind installed. In System Settings, Photo Stream is enabled, of course.

???

Thanks a lot for a good idea!

Peter