After Upgrade to 4.3-1: driver for horde_auth not found

I did a retry on this matter - to no avail. Every output on the suggested queries seems to be correct:

docker ps
CONTAINER ID        IMAGE                                                      COMMAND                  CREATED             STATUS              PORTS                                           NAMES
d98b746d91af        docker.software-univention.de/ucs-appbox-amd64:4.3-3       "/sbin/init"             13 minutes ago      Up 13 minutes       0.0.0.0:40000->80/tcp, 0.0.0.0:40002->443/tcp   confident_murdock
bf611ffb1b9d        docker.software-univention.de/collabora:4.0.8.2            "/bin/sh -c 'bash st…"   2 hours ago         Up 2 hours          0.0.0.0:9980->9980/tcp                          hopeful_kapitsa
e4881e1facd3        docker.software-univention.de/dudle:1.2                    "/start.sh"              6 days ago          Up 6 days           0.0.0.0:40001->80/tcp                           thirsty_poitras
4fe60a11c0a4        docker.software-univention.de/owncloud-owncloud:10.2.1-1   "/usr/bin/entrypoint…"   6 weeks ago         Up 9 days           0.0.0.0:40008->8080/tcp                         owncloud_owncloud_1
0b5780f59110        docker.software-univention.de/owncloud-redis:10.2.1-1      "/usr/bin/entrypoint…"   6 weeks ago         Up 9 days           6379/tcp                                        owncloud_redis_1
944d7f38d863        docker.software-univention.de/ucs-appbox-amd64:4.3-0       "/sbin/init"             16 months ago       Up 9 days           0.0.0.0:40006->80/tcp                           musing_goldstine
7ce77cd99a4f        docker.software-univention.de/wordpress:4.9.4

,

ucr get appcenter/apps/horde/container
d98b746d91af39c0f303e8c046a871becb3aeda5a3ac2c6563150a3979e3b95e

,

univention-app shell horde ucr search --brief version
repository/mirror/version/end: <empty>
repository/mirror/version/start: <empty>
repository/online/component/.*/version: <empty>
repository/online/component/4.3-3-errata/version: 4.3
repository/online/component/horde_20190925130937/version: current
update/umc/nextversion: true
version/erratalevel: 520
version/patchlevel: 3
version/releasename: Neustadt
version/version: 4.3

,

univention-app shell horde cat /etc/horde/horde/conf.d/10-ucs.php| grep auth
$conf['mailer']['params']['auth'] =  true;
$conf['auth']['admins'] = array();
$conf['auth']['params']['app'] = 'imp';
$conf['auth']['driver'] = 'application';
$conf['auth']['redirect_on_logout'] = '/';

and

univention-app shell horde cat /etc/horde/horde/conf.d/10-ucs.php| grep ldap
$conf['ucs']['ldaprdn'] = 'cn=horde-44667944,cn=memberserver,cn=computers,dc=cats-net,dc=intranet';
$conf['ucs']['ldappass'] = 'geheim';
$conf['ucs']['ldapbase'] = 'dc=cats-net,dc=intranet';
$conf['ucs']['ldaphost'] = 'lion.cats-net.intranet';
$conf['ucs']['ldapport'] = 7389;

As mentioned already: Removing and reinstalling horde does not help. Clicking on “Open” tries to load horde from “localhost” (which of course fails). If I replace localhost with its dns name the “driver for horde_auth not found” message pops up.

Any ideas ?

Martin