First, it would be helpful to get some screenshots and/or the text of the error these browsers are getting.

It might be the JavaScript version, but it might also be the browser's document model (the API of the browser) that is less functional than newer version of Internet Explorer.

As far as I could dig from the various forums of AJAX.NET (formerly Atlas) it should work with Internet Explorer 5.0 and above.

It might degrade itself functionality-wise due to the old version of the browser but it should work.