Blank window with download option

Hi Guys

Opening this url: 'window.open('https://myServerName.intra.domain.ch/ibis/rest/rc/TCRSP-getCable/KTN-119196:1K') returns a blank window with download option. The same call in chrome works and is displayed in the new window.
However, this works: 'window.open('http://www.africau.edu/images/default/sample.pdf')'.

Any ideas or pointers please?
Many thanks 
Jaco