open_basedir not effective

[mcnesium]

I am trying to use the Piwik PHP API for the Piwik Wordpress Plugin as discussed here.

The webspace still runs on Ubuntu 12.04 LTS which features PHP 5.3, so I updated PHP to 5.4 manually as described here.

Now running PHP Version 5.4.40-1+deb.sury.org~precise+4 on Ubuntu 12.04 LTS and Parallels Plesk Panel v. 12.0.18

Piwik is located at /var/www/vhosts/server.domain/analytics/ and the Wordpress blog at /var/www/vhosts/wordpress.domain/httpdocs/ - so I added the Piwik path to the open_basedir setting in the PHP Settings of the wordpress webspace like this: {WEBSPACEROOT}/:/var/www/vhosts/server.domain/analytics/:{TMP}/

Then I set define('WP_DEBUG', true ); and got the following error message, when trying to set the Piwik Path for the PHP API in the Plugin settings:

Warning: file_exists(): open_basedir restriction in effect. File(/var/www/vhosts/server.domain/analytics/index.php) is not within the allowed path(s): (/var/www/vhosts/wordpress.domain/:/var/www/vhosts/server.domain/analytics/:/tmp/) in /var/www/vhosts/wordpress.domain/httpdocs/content/plugins/wp-piwik/classes/WP_Piwik/Request/Php.php on line 26

Note that the required path actually is in the allowed paths, so the error message is colliding with itself. Now my question is: is this a Plesk related issue? Plesk still shows PHP-Version 5.3 in the Websites & Domains view of the webspace settings, so maybe there is something wrong with Plesk and the new PHP version.

What can I do?