Oops, the latest version of PHP breaks a few things on Plesk 8.1.x and Plesk 8.2.x installations. It propably affects other installations as well, but I stopped testing since I was only interested in Plesk 8.2.x and later.
According to atomicrocketturtle the problem will be fixed in Plesk 8.3. We will just have to wait and see.
On rackerhacker‘s blog I found a very elegant solution to the problem that was provided by Kevin M.
I repeat the entry from rackerhacker‘s blog:
Thereâ€™s a few issues with PHP 5.2.5 and the version of Horde that is bundled with Plesk 8.1.x and 8.2.x. The PHP include paths that appear in the Apache configuration generated by Plesk conflict with the PHP installation, and that causes the Horde webmail interface to segmentation fault.
To fix the problem, create a file called
/etc/httpd/conf.d/zz050a_horde_php_workaround.conf and put the following inside it:
php_admin_value include_path "/usr/share/psa-horde/lib:/usr/share/psa-horde:/usr/share/psa-horde/pear:."
Reload the Apache configuration and your Horde installation should work properly with PHP 5.2.5.