Hi Arnd,
Well since it’s working on your laptop I assume with at least W7 and not working on Vista with IE9 you definitely could be on to something with your conclusion.
Personally I’ve never used Vista, but at least at the moment I am inclined to agree with your conclusion about it and IE9.
Are your Xibo for Windows clients in 1.7.5 version? As well as CMS? (1.7.4 should be fine too, 1.7.5 was released pretty much today)
Did you try to set this to IE9 (on your Vista PC)? Enabling HTML5 in the Windows .net Player