Well, this is very weird. I've been using this plugin for months without issue, and it's great ! (Definitely the best option for page tabs, and I tried them all). But I just noticed today that my site wasn't displaying properly in IE. I work mainly in Chrome, with Firefox/Firebug on the side - plugin works fine in these browsers, only IE is the problem (site was completely unusable - looking like my child theme STYLE.CSS was not loading at all).
I use a lot of plugins, so disabled them all at first -> site fine. So obviously a plugin issue.
So I started re-enabling them and it's definitely this plugin, as I can disable/re-enable repeatedly to see my site breaking (in IE only, Chrome/FF are OK).
My guess is that this clashes with another plugin, but cos I need all my others I'll have to deactivate this for now as it's only cosmetic. If I find more info, or workout which plugin this clashes with, then I'll update here.
I wonder if it was the Windows 7 updates from earlier today ? (Only a hunch - no evidence for this).
Looks like this affects another of my plugins too - another I've been using for months - "WP Pagenavi Style". Again, only in IE, and again I can disable/re-enable repeatedly to see my site breaking. So now I have these 2 plugins disabled and my site works fine; re-enabling 1 or both of them 'breaks' things again in IE.