Hi,
That is very strange it seems like a bug in YUI tabs that only affects IE 7. It does not happen in IE 8, Chrome or Firefox.
There is no script error either, if you click one of the sub tabs besides Main under security it fixes itself, you can select main again after that and it is fine.
Not really sure of any solution for it since it seems to be an issue with YUI and it doesn't involve any script error.
Best,
Joe