This is also happening to me from time to time. But in my case, I added users through Qluman, as described in the docs. These are the last lines of the /var/log/auth.log file on the FE node, right after the login attempt.
...
sshd[9118]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=10.0.2.159 user=********* sshd[9118]: pam_sss(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=10.0.2.159 user=********* sshd[9118]: pam_sss(sshd:auth): received for user *********: 7 (Authentication failure) sshd[9118]: Failed password for ********* from 10.0.2.159 port 40398 ssh2
I've omitted the username and the timestamps and hostname columns from the lines above. Does anyone have any clue what might be going on? Do I need to show anything else?
--- Qlustar Version: *12* Head-Node Kernel Version: *5.13.19-051319-generic* QluMan Server Version: *12.0.7.5* Image (FE) Kernel Version: *5.4.209-ql-generic-12.0-19* Image (FE) Version: *Qlustar 12.0.1.0-b548f1441*