Unable to use sftp with root account

[[EMAIL PROTECTED]]

Hello, I am used to connect to my LAN server using sftp (with tectia client), logging in as "root".
My server is a Debian testing updated day by day.
Since 2 or 3 days I am unable to connecto to the server as "root".
If I try to connect with another account I get in; seems that some software are blocking me if I try root access.
I looked in /etc/ssh/sshd_config and pam config but nothing's wrong. I also tried to restore old configuration but nothing.
Any idea ?

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/etc/ssh/sshd_config

Port 22
Protocol 2
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_dsa_key
UsePrivilegeSeparation yes
KeyRegenerationInterval 3600
ServerKeyBits 768
SyslogFacility AUTH
LogLevel INFO
LoginGraceTime 120
PermitRootLogin yes
StrictModes yes
RSAAuthentication yes
PubkeyAuthentication yes
IgnoreRhosts yes
RhostsRSAAuthentication no
HostbasedAuthentication no
PermitEmptyPasswords no
ChallengeResponseAuthentication no
X11Forwarding yes
X11DisplayOffset 10
PrintMotd no
PrintLastLog yes
KeepAlive yes
AcceptEnv LANG LC_*
Subsystem sftp /usr/lib/openssh/sftp-server
UsePAM yes
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/etc/pam.d/login

auth       requisite  pam_securetty.so
auth       requisite  pam_nologin.so
session       required   pam_env.so readenv=1
@include common-auth
auth       optional   pam_group.so
@include common-account
@include common-session
session    required   pam_limits.so
session    optional   pam_lastlog.so
session    optional   pam_motd.so
session    optional   pam_mail.so standard
@include common-password
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