Clicking on blogpost causes messed up markup

Likely two extra div tags in your footer — see lines 143-144 from this:

https://validator.w3.org/check?uri=http%3A%2F%2Fwww.andyschoen.com%2F2012%2F08%2F24%2Fvakantie-finland-2012%2F&charset=%28detect+automatically%29&doctype=Inline&group=0

if this happens with every single post, the error might be in single.php or comments.php of your theme.

how did you integrate the slider into the single posts?

@wpyogi: how do I fix this?

@alchymyth: It happens with the all my blogpost (2 in total). I have added the slider in the editor with the standard wordpress “upload/add” button above the buttons from the editor with which you can add multiple images and put them together in a galary.

if you have edited single.php in any way, please paste the full code into a pastebin and post the link to it here – https://codex.www.ads-software.com/Forum_Welcome#Posting_Code

as far as I could see, there is a </div> too many somewhere before the call of the sidebar.

This is what is in single.php
Single.php

What do I need to edit in it?

how (through what kind of edits in what template) did you get the slide show to show in single posts (this is not a default for your theme).

I use the plugin RPS Image Gallery. I made no adjustments of this kind to my template.

But the problem already existed before I added these slideshows to the posts.

what is the code in header.php?

header.php

Is that relevent for something in a blogpost?

Was the single.php file ok?

single.php was OK, as in the original download.

everything before the sidebar call could be relevant; just checking, as this is not in the original download, it has to be something either through your edits or through added plugins.

you seem to have replaced the original section in header.php:

<?php if ( get_theme_mod( 'header_image' ) ) : ?>
	<div id="header-image">
	<img src="<?php header_image() ?>" width="950" height="200" alt="" />
	</div>
	<?php endif; ?>

with your <!-- Begin slideshow --> code, with one opening div

<div id="slideshow">

and two closing divs

</div>
</div>

your choice:
remove either line 62 or line 63 of your header.php pastebin code.