Takayuki Miyoshi on "[Plugin: Contact Form 7] validation message for fields"

It is due to the theme. I haven't seen such an awful one!

http://www.liofbedrijvencentra.nl/wp-content/themes/makalu/cache/all.css
Line #9320

div.wpcf7 .wpcf7-not-valid-tip {
  display: none !important;
}

Line #9337

#footer-sidebars .wpcf7 .wpcf7-form > p .wpcf7-form-control-wrap .ajax-loader {
  display: none;
}

You at least need to remove these lines to get Contact Form 7 to work correctly, but I'd rather suggest to change theme. You shouldn't buy from ThemeForest.