Hi,
Thanks for fast reply.
I tried it on a Windows 7 Ultimate 64-bit with IE 9.0.8112.16421 and a Windows 7 Home Premium 64-bit with the same IE, both show the same problem.
Both with cleaned cache and up-to-date ATI graphics drivers.
Another Windows 7 PC (same IE version) doesn't show the problem at all.
I wonder if there is a way to investigate this further
EDIT: the only thing I discoverd is that when I turn off the grey thing behind the lightbox (which darkens the page) the problem disappears completely