Kirby adds port number twice to url of resized image

In my environment-specific config file, I set

c::set('url', 'http://salon-catherine.dev:3000');

On one page, I output a bunch of images like so:

<?php $n=0; foreach($carousel_images as $image): $n++; ?>
<div class="item<?php if($n==1) echo ' active' ?>">
  <div class="image-wrapper">
    <img src="<?php echo $image->resize(c::get('carousel_max_width'), c::get('carousel_max_height'))->url() ?>" alt="<?php echo html($image->title()) ?>" class="img-responsive" />
    <?php if (!$image->caption()->empty()): ?>
      <div class="carousel-caption">
        <?php echo kirbytext($image->caption()) ?>
      </div>
    <?php endif; ?>
  </div> <!-- .image-wrapper -->
</div>
<?php endforeach ?>

Which produces the following output. Notice how the middle item has the port number attached 2 times. The image is obviously not displayed:

<div class="item">
  <div class="image-wrapper">
    <img src="//salon-catherine.dev:3000/thumbs/4-umstyling1-3028dac80672c8bb3c2fcba3a1bc39fa.jpg" alt="" class="img-responsive" />
    <div class="carousel-caption">
      <p>unsere Arbeit</p>
    </div>
  </div><!-- .image-wrapper -->
</div>
<div class="item">
  <div class="image-wrapper">
    <img src="//salon-catherine.dev:3000:3000/thumbs/5-umstyling1-bc92b1f306404f68af70f5e37b37bbbd.jpg" alt="" class="img-responsive" />
    <div class="carousel-caption">
      <p>unsere Arbeit</p>
    </div>
  </div><!-- .image-wrapper -->
</div>
<div class="item">
  <div class="image-wrapper">
    <img src="//salon-catherine.dev:3000/thumbs/6-umstyling1-ded7d33a37e2b53d9076bb7789d677a6.jpg" alt="" class="img-responsive" />
    <div class="carousel-caption">
      <p>unsere Arbeit</p>
    </div>
  </div><!-- .image-wrapper -->
</div>

Here comes the weird part: When I remove the .carousel-caption part from the PHP code, the problem no longer occurs:

<?php $n=0; foreach($carousel_images as $image): $n++; ?>
<div class="item<?php if($n==1) echo ' active' ?>">
  <div class="image-wrapper">
    <img src="<?php echo $image->resize(c::get('carousel_max_width'), c::get('carousel_max_height'))->url() ?>" alt="<?php echo html($image->title()) ?>" class="img-responsive" />
  </div> <!-- .image-wrapper -->
</div>
<?php endforeach ?>

This works but is obviously not an option.

Another observation: If I call $image->url() directly, omitting the resize() part, the problem doesn’t occur either.

Hm. Generally Kirby should detect the port, so what happens if you set the url option to http://salon-catherine.dev without the port?

The page loads but BrowserSync, which I use while developing, won’t work anymore.