ecryptfs-mount-private does not work after su - user

When I do a "su - user" as user root and this user has an encrypted home,
ecryptfs-mount-private fails with "No such file or directory":

root@wupp:~# su - framstag
Signature not found in user keyring
Perhaps try the interactive 'ecryptfs-mount-private'

framstag@wupp:~: ll
lrwxrwxrwx framstag users - 2016-09-13 17:40:15 Access-Your-Private-Data.desktop -> /usr/share/ecryptfs-utils/ecryptfs-mount-private.desktop
lrwxrwxrwx framstag users - 2016-09-13 17:40:15 README.txt -> /usr/share/ecryptfs-utils/ecryptfs-mount-private.txt

framstag@wupp:~: ecryptfs-mount-private
Enter your login passphrase:
Inserted auth tok with sig [8d3041402fc70c61] into the user session keyring
mount: No such file or directory

framstag@wupp:~: ll
lrwxrwxrwx framstag users - 2016-09-13 17:40:15 Access-Your-Private-Data.desktop -> /usr/share/ecryptfs-utils/ecryptfs-mount-private.desktop
lrwxrwxrwx framstag users - 2016-09-13 17:40:15 README.txt -> /usr/share/ecryptfs-utils/ecryptfs-mount-private.txt

In contrast, login via ssh works as expected:

root@wupp:~# ssh framstag@localhost
framstag@localhost's password:
Welcome to Ubuntu 16.04.1 LTS (GNU/Linux 4.4.0-36-generic x86_64)

 * Documentation: https://help.ubuntu.com
 * Management: https://landscape.canonical.com
 * Support: https://ubuntu.com/advantage
Last login: Wed Sep 14 13:04:17 2016 from 127.0.0.1

framstag@wupp:~: ll
drwxr-xr-x framstag users - 2016-08-17 12:27:18 Downloads

root@wupp:~# mount | grep crypt
/local/home/.ecryptfs/framstag/.Private on /local/home/framstag type ecryptfs (rw,nosuid,nodev,relatime,ecryptfs_fnek_sig=ff00eef45bff6817,ecryptfs_sig=8d3041402fc70c61,ecryptfs_cipher=aes,ecryptfs_key_bytes=16,ecryptfs_unlink_sigs)

root@wupp:~# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 16.04.1 LTS
Release: 16.04
Codename: xenial

root@wupp:~# dpkg -l | grep ecryptfs-utils
ii ecryptfs-utils 111-0ubuntu1.1 amd64 ecryptfs cryptographic filesystem (utilities)

--
Ullrich Horlacher Server und Virtualisierung
Rechenzentrum TIK
Universitaet Stuttgart E-Mail: <email address hidden>
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