Plesk 8 Horde: redirect.php is unavailable

[Streicher4]

After the login at horde, i get the following error message:

http://webmail.mydomain.info/imp/redirect.php is not available

How can I fix this?

I am running SUSE 9.3, PHP5, MySQL4 and Apache 2 with Plesk 8 and Horde 3