Hello,
We have just checked out your Website and the issue is that the .slider-parallax-inner Container is not currently supported with the Header + Top Bar Combination but only with Transparent Headers. So please remove it .slider-parallax-inner Container from your #slider Element and this issue will be automatically resolved. So replace your existing #slider Code with the following:
<section id="slider" class="slider-parallax" style="height:800px">
<div class="container clearfix">
<div class="hero-intro no-fade">
<div class="col-md-12 dark center nobottommargin" data-animate="fadeIn">
<div class="emphasis-title">
<h1>Je auto <span id="js-rotating">makkelijk, snel, goed, professioneel</span> laten wassen</h1>
</div>
</div>
<form action="" method="post" id="postcode-form" novalidate="novalidate">
<div class="row">
<div class="col-xs-5 col-md-offset-2 dark center nobottommargin" data-animate="fadeIn">
<input type="text" id="postcode" name="postcode" maxlength="6" required>
</div>
<div class="col-xs-5">
<a href="javascript:void(0)" id="postcodeCheck" class="button button-3d button-large button-rounded button-wyper">Check Postcode</a>
</div>
</div>
</form>
<div class="col-xs-5 col-md-offset-2" id="errorBox"></div>
</div>
</div>
</section>
Let us know if we can help you with anything else or if you find any further issues.