...no one on staff is able to reproduce this issue.
What version of IE are they using? Please make sure they trying it on version 8 and/or 9.
I have tested 3 more computers running Windows and IE (1 with IE8 and 2 with IE9)... none were able to log on to the cPanel.
That's a total of 5 machines I've tested. Two are mine, 3 are unrelated and belong to other people. The only things they have in common is that they all run IE 8 or 9 and they all are unable to access your cPanel.
We haven't changed any code on our end, therefore it's not an issue with our software
Are you sure that on, or shortly after, Feb 25th (the last time I successfully logged in) something wasn't changed?
Maybe someone updated some of the HTML, Style Sheets or Javascript. Possibly getting rid of some of the old outdated and deprecated HTML tags and attributes? Or possibly some new Javascript, or Javascript that is browser specific? Some of the newer features of the codes may not be supported by the older browsers, they still need that deprecated code.
I see that the log-in page tests for IE browsers earlier than version 7 and uses separate code if true. Maybe you also need something like that for IE 8 and 9.