It looks like your WordPress installation or web server is preventing direct access to PHP files (which will break WP-Juicebox).

If you use the BulletProof Security plugin, then this will almost certainly be the cause of your problem and you will need to add a skip/bypass rule to the .htaccess file in your root directory as follows:

# Juicebox skip/bypass rule
RewriteCond %{REQUEST_URI} ^/wp-content/plugins/wp-juicebox/ [NC]
RewriteRule . - [S=13]

If you do not use the BulletProof Security plugin, then please check your .htaccess file for an entry such as:

<Files *.php>
    deny from all
</Files>

If you find such an entry, either comment it out or remove it and WP-Juicebox should work as expected.

If I go directly to the PHP file which generates your gallery's XML data, an error 404 (file not found) is displayed.
http://www.foto.pelles.ch/wp-content/pl … llery_id=7

This suggests that either the file does not exist on your web server, or that access to the file is being blocked.
This is the reason why both your gallery settings pop-up window (in the backend) and the gallery (in the frontend) do not display. They require direct access to the 'jb-config.php' and 'config.php' files respectively.
(This has not changed between v1.4.0.1 and v1.4.2.0.)

Please check the following:
(1) Check that all the WP-Juicebox files (including 'config.php' and 'jb-config.php') are present on your web server.
(2) Check that the permissions on these files are not too restrictive. (Permissions of 644 should be fine.)

Also, you could try checking with your web host to see if they have made any changes recently which would be affecting things this way.

The '7.xml' file (and all other numerical XML files in the 'uploads/juicebox/' directory) store settings for each gallery but they are not the actual XML files used by Juicebox when the galleries are displayed.

The XML files used by Juicebox when the galleries are displayed are generated dynamically by the 'config.php' file (with a 'gallery_id' query string so that Juicebox knows which numerical XML file to take the settings from) and it is because the plugin cannot access PHP files directly that the "Config XML file not found." message is displayed.

Try temporarily deactivating all plugins (other than WP-Juicebox) and reactivate them one-by-one (checking the functionality of your Juicebox galleries after each one) to see if you can find a conflicting plugin.

The problem seems to be one of security so if you have any security-related plugins, I would try disabling them first.

The problem is that your web server is denying direct access to PHP files.
If you contact your web host, they may be able to help or point you in the right direction.

You could certainly load your galleries into iframes but you could also embed your galleries directly into your WordPress pages or posts using the baseUrl method of embedding as documented here.
Essentially, you would create a Juicebox-Pro gallery using JuiceboxBuilder-Pro, upload your entire Juicebox gallery folder (not just the contents) to your web server and then paste the baseUrl embedding code into the body of your WordPress page or post (ensuring that the method of entry is 'Text' rather than 'Visual').
It does not matter where on your web server you upload your gallery folder to as long as the two paths in the embedding code (the path to the 'juicebox.js' file and the baseUrl itself) are correct.