Turning on debugging in IE9 and then hitting "Edit" ends up with this in the console:
SCRIPT16389: Unspecified error. load.php?debug=false&lang=en&modules=ext.visualEditor.core%2Cdata%2Cicons- vector%7Cext.visualEditor.viewPageTarget.icons-vector%7Crangy&skin=vector& version=20140408T180250Z&*, line 642 character 159
The line that is highlighted (running with "Format JavaScript" checked) is:
this.nativeSelection.addRange(getNativeRange(range).cloneRange());
Version: unspecified
Severity: normal