The php version for the wordpress installation is wrong on the server (web.de). At the provider (web.de) i can't find any possibility to change the php version.
Your problem is that by default, web.de uses PHP4 to interpret .php files. PHP5 is installed, you can try by adding three files:
test.php5 with content <?php phpinfo() ?>test.php4 with content <?php phpinfo() ?>test.php with content <?php phpinfo() ?>Calling them all from your browser, you will find out that .php and .php4 are interpreted using php 4, while .php5 is interpreted using php 5!
You can change this behaviour by adding a .htaccess file to your wordpress directory with the following content:
Type application/x-httpd-php5 .php