It appears that the SS4200 software is compatible with IE9, even on my 64 bit Win 7 system.
I downgraded to IE8, and got the same error window when trying to open the NAS. Next I reinstalled the NAS software and voila, it worked.
Finally I re-upgraded to IE9. Again I got the error. But a quick reinstall of the NAS software and I was back in.
So while IE9 is fully compatible, it appears that the NAS software MUST be reinstalled AFTER any install/upgrade of your web browser (kind of makes sense).
Everything's working great now.