We found that the taxonomy controls don’t load in IE11 with Rhythmyx versions 7.2 or 7.3. Had to follow the instructions here http://stackoverflow.com/questions/20285864/jstree-xml-data-plugin-not-working-in-ie11
Basically in jquery.jstree.rev236-taxonomy.js find the line:
if (window.ActiveXObject)
and replace it with:
if (window.ActiveXObject !== undefined || "ActiveXObject" in window)