Thank you for filing this bug, and sorry for the lack of responses.

The problem is that there are two secrets.tdb databases:

Preparing to unpack 
/samba-libs_2%3a4.1.6+dfsg-1ubuntu2_amd64.deb ...
secrets.tdb exists in /var/lib/samba and /var/lib/samba/private, aborting samba-libs preinst
rename one of them to allow the install/upgrade to continue
http://bugs.debian.org/726472
/var/lib/samba:
ОтПгП 716
drwxr-xr-x  10 root root         4096 апр.  20 08:01 .
drwxr-xr-x  78 root root         4096 апр.  20 08:13 ..
-rw-------   1 root root        36864 сеМт.  3  2012 account_policy.tdb
-rw-------   1 root root          696 сеМт.  3  2012 group_mapping.tdb
drwxr-xr-x   2 root root         4096 апр.   5 00:26 netlogon
drwxr-x---   2 root root         4096 апр.   4 15:35 ntp_signd
-rw-------   1 root root        36864 апр.  12 00:24 passdb.tdb
drwxr-xr-x   6 root root         4096 апр.   4 15:35 private
drwxr-xr-t   4 root root         4096 апр.   5 00:30 profiles
-rw-------   1 root root       106496 апр.   5 00:59 registry.tdb
drwxr-xr-x   2 root root         4096 апр.   5 00:26 root
-rw-------   1 root root        45056 апр.   5 00:26 secrets.tdb
-rw-------   1 root root        36864 Ќарта  4 16:14 share_info.tdb
drwxrwx---+  3 root    3000000   4096 апр.   2 13:07 sysvol
drwxrwx--T   2 root sambashare   4096 яМв.  28  2013 usershares
-rw-r--r--   1 root root       421888 апр.   5 01:22 winbindd_idmap.tdb
drwxr-x---   2 root root         4096 апр.   4 15:35 winbindd_privileged

/var/lib/samba/private:
ОтПгП 10904
drwxr-xr-x  6 root root    4096 апр.   4 15:35 .
drwxr-xr-x 10 root root    4096 апр.  20 08:01 ..
-rw-r--r--  1 root root    2270 апр.   2 13:08 dns_update_list
-rw-------  1 root root 1286144 апр.   2 13:07 hklm.ldb
-rw-------  1 root root 1286144 апр.   2 13:07 idmap.ldb
-rw-r--r--  1 root root      92 апр.   2 13:08 krb5.conf
srwxrwxrwx  1 root root       0 апр.   4 15:35 ldapi
drwxr-x---  2 root root    4096 апр.   4 15:35 ldap_priv
-r--r--r--  1 root root     223 апр.   2 13:08 named.conf.update
-rw-------  1 root root 1286144 апр.   2 13:07 privilege.ldb
-rw-------  1 root root     696 апр.   2 13:08 randseed.tdb
-rw-------  1 root root 4251648 апр.   2 13:08 sam.ldb
drwx------  2 root root    4096 апр.   2 13:08 sam.ldb.d
-rw-------  1 root root     696 апр.   4 15:35 schannel_store.tdb
-rw-------  1 root root    1102 апр.   2 13:08 secrets.keytab
-rw-------  1 root root 1286144 апр.   2 13:08 secrets.ldb
-rw-------  1 root root  430080 апр.   2 13:08 secrets.tdb
-rw-------  1 root root 1286144 апр.   2 13:07 share.ldb
drwxr-xr-x  3 root root    4096 апр.   2 13:08 smbd.tmp
-rw-r--r--  1 root root     955 апр.   2 13:08 spn_update_list
drwxr-xr-x  2 root root    4096 апр.   2 13:08 tls
dpkg: error processing archive /var/cache/apt/archives/samba-libs_2%3a4.1.6+dfsg-1ubuntu2_amd64.deb (--unpack):
,..


The upstream debian bug http://bugs.debian.org/726472 has a lot of details about this problem.

Are you still affected by this? It looks like both secrets.tdb databases should be compared and a decision be made about which one to use. The current and expected location is in /var/lib/samba/private.