Strange log on behaviour

I have an xwiki on the wiki farm. Users sometimes experience the following behaviour: (1) Go to site, see a log on screen (2) Username and password entered - taken to an html style sheet (I think) - this shows up as a new window in IE, takes over existing window in Firefox (3) Click back in case of Firefox, or close window for IE (4) Taken to "normal" log-on screen see screen shots here: http://www.thecolehouse.org.uk/screenshots.htm (trying to get screen shoot of css but all machines to hand have got password remembered and so this behaviour is skipped) This is off-putting for non-catechise new users cause they log on and in their eyes get a page of gobbledygook and then give up. Can anyone advise? Thanks