I also had this problem and solved it by changing a part of the code in the file script.js (contact-form-7/includes/js/script.js)
Replace the complete function $.fn.wpcf7NotValidTip with the following code (taken from an older version of Contact-Form-7):
$.fn.wpcf7NotValidTip = function(message) {
return this.each(function() {
var into = $(this);
into.append('<span class="wpcf7-not-valid-tip">' + message + '</span>');
$('span.wpcf7-not-valid-tip').mouseover(function() {
$(this).fadeOut('fast');
});
into.find(':input').mouseover(function() {
into.find('.wpcf7-not-valid-tip').not(':hidden').fadeOut('fast');
});
into.find(':input').focus(function() {
into.find('.wpcf7-not-valid-tip').not(':hidden').fadeOut('fast');
});
});
};
It works for me.
Roland