No images too

Now I’ve enabled debugging in config.php of plugin and get this error:

mPDF error: IMAGE Error (http://www.foo.de/wp-content/uploads/2014/08/6.jpg): Error parsing image file – image type not recognised, and not supported by GD imagecreate

Path is valid, I can request and view the images in browser. They are all JPGs.
No password protection.

It may be important to know, that this is a multisite installation. Here is the .htaccess:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ – [L]

# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ – [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]
</IfModule>
# END WordPress

I’m not sure, what the line with wp-content is meaning.

Thank you, Florian.

If I find a solution, I’ll provide it here.