i think i got a good workaround from parallels support.
You can turn off Deprecated warnings by adding this line into /etc/httpd/conf.d/zzz_horde_vhost.conf:
<IfModule mod_php5.c>
..
php_admin_flag error_reporting "E_ERROR & E_CORE_ERROR & E_USER_ERROR & E_COMPILE_ERROR"
..
</IfModule>...