This is the code that Joomla (3.2) shows:

<h2>Foto&#039;s finale clubkampioenschap bandstoten op 27 april 2013</h2>
<p><b>Klik op een foto om deze vergroot te zien. Vervolgens kunt u verder of terug bladeren.</b></p>
<p><b>{gallery}27april2013{/gallery}</b></p>
<h2>Foto&#039;s finale clubkampioenschap libre op 15 december 2012</h2>
<p><b>Klik op een foto om deze vergroot te zien. Vervolgens kunt u verder of terug bladeren.</b></p>
<p>{gallery}15december2012{/gallery}</p>
<h2>Foto&#039;s finale clubkampioenschap libre op 28 april 2012</h2>
<p><b>Klik op een foto om deze vergroot te zien. Vervolgens kunt u verder of terug bladeren.</b></p>
<p><b>{gallery}28april2012{/gallery}</b></p>
<p> </p>

As you can see there are three folders. The second (15december2012) only shows as:

{gallery}15december2012{/gallery}

There are no spaces in whatever filename. It doesn&#039;t make any difference where I put the code. 1st, last... it doesn&#039;t work.

Is there anybody that might have a clue what&#039;s going on?

Hi. Check that the third folder does exist in your server and that it contains valid image files. Also check that these files are named properly ( lowercase letters, no spaces and other special characters ).

Thanks for the answer.
Although the directory exists on the server and the file names are correct, the problem was solved after I loaded all the pictures in Photoshop and used the menu item "Save for Web".
Apparently something was wrong in the picture files that was corrected by Photoshop.

Your answer triggered me to focus on the pictures themselves.
So... problem solved.

Thanks

You are welcome.