The popup functionality causes a script error in IE8, if the popup window is already open. As a result the link's default behaviour is executed instead - as ' return false; ' never is reached.
The problem exists in line: 188 // if(w.blur) w.focus();
In IE8 blur and focus isen't defined - secound time a popup link is called and therefore the logic breaks.
//An alternative: (test if w.blur is defined)
script.append("'); if( jQuery.isFunction( w.blur ) && w.blur )
").append(" return false;");