Blank window in second CSH call

Hi, in IE6, with a CSH window open, when a user makes another help call it should replace the first topic in the help window. This always used to happen and now it doesn't. User gets a blank window on the second help call, which has the title "about:blank". Firefox v2 works OK. Using RH7.

Has anyone come across this? Anyone aware of any IE updates which might affect the window behaviour? The developers claim not to have changed anything.