pam_oath/sshd debug logging on Debian 10

[Felix Natter]

hello oath-toolkit-help,

I am trying to use pam_oath.so on Debian 10 for SSH access:

ii  liboath0       2.6.1-1.3          amd64        OATH Toolkit Liboath library
ii  libpam-oath    2.6.1-1.3          amd64        OATH Toolkit libpam_oath PAM 
module
ii  oathtool       2.6.1-1.3          amd64        OATH Toolkit oathtool 
command line tool
ii  openssh-server 1:7.9p1-10+deb10u2 amd64        secure shell (SSH) server, 
for secure access from remote machines

/etc/ssh/sshd_config:
ChallengeResponseAuthentication yes
UsePAM yes
match group sudo
   PubkeyAuthentication yes
   PasswordAuthentication yes
   AuthenticationMethods keyboard-interactive

Prepended to /etc/pam.d/ssh:
auth sufficient pam_oath.so debug usersfile=/etc/users.oauth window=10 digits=6 
verbose=1
#auth [success=ok new_authtok_reqd=ok default=die] pam_oath.so debug 
usersfile=/etc/users.oauth window=10 digits=6

/etc/users.oath:
HOTP felix - 00

(this user is in the sudo group)

I am using a zero key, because according to the documentation [1], this
causes the first OTP to be 328482 (for simplicity, no
FreeOTP/authenticator involved).

[1] https://www.nongnu.org/oath-toolkit/pam_oath.html

Despite the debug flag for pam_oath.so, the only thing I see in
/var/log/auth.log is this:

May 16 19:46:36 delllaptop sshd[1380]: error: PAM: Authentication failure for 
felix from 192.168.178.21
May 16 19:46:37 delllaptop sshd[1380]: error: PAM: Authentication failure for 
felix from 192.168.178.21
May 16 19:46:38 delllaptop sshd[1380]: error: PAM: Authentication failure for 
felix from 192.168.178.21

/var/log/debug contains mostly kernel- und no PAM-messages. There is
also no PAM[-oath] logging on the client (I entered "328482" 3x):

$ ssh dellnotebook
One-time password (OATH) for `felix': 
One-time password (OATH) for `felix': 
One-time password (OATH) for `felix': 
felix@dellnotebook: Permission denied (keyboard-interactive).

So how can I configure debug logging to find out what the problem is?

Many Thanks! Best Regards,
-- 
Felix Natter