Na documentação on-line :
What are the default user name and password for the administrator?
There are none. Since Horde supports a broad variety of different authentication backends and you are free to choose any of them for your Horde installation, the selected backend decides which are valid login credentials. And we didn't add an authentication independent backdoor to Horde, for obvious reasons. Instead, you can turn any valid and authenticated user into an administrator by adding his user name to the administrator list in Horde's configuration. Make sure to use user names exactly like they are used inside Horde, i.e. with or without a domain name, and considering any user name hooks that you enabled.
Para isso, edite o arquivo /config/config.php
adicionando a seguinte linha:
$conf['auth']['admins'] = array('your_admin_user');
Isso deve permitir que você faça o login. De lá, você deve poder configurar todo o resto.