Unable to SSH with Kerberos user

Hello!
I am trying to set up a Kerberos server and a client for the first time,
both using Xubuntu 18.04. I created a normal user `joe' and I am able
to successfully do, from the client:

$ kinit joe
Password for [email protected]:
$ klist
Ticket cache: FILE:/tmp/krb5cc_1000
Default principal: [email protected]

Valid starting       Expires              Service principal
25/01/2020 16:10:42  26/01/2020 02:10:42  krbtgt/[email protected]
	renew until 26/01/2020 16:10:28

Despite the client and server being in the same LAN, when I enter the
password a long wait of several seconds occurs, before the prompt is back
again. I would now like to ssh into the Kerberos server from the client,
as `joe', without being prompted again for a password:

$ ssh joe@<server_FQDN>

However, the password is asked here, despite the TGT shown above, and
even with the correct password the permission is denied.

What could be wrong with this configuration? Also, I still did not
understand the role of the keytab in this operation. Is it necessary?

Note that my user (in the Xubuntu system of the client) has not the name
`joe', as shown in the logs below: `joe' only belongs to Kerberos.

Log of ssh with `-vvv' option:	https://pastebin.com/DSueXmf0
Client /etc/ssh/ssh_config:	https://pastebin.com/14FWX5ye
Client /etc/krb5.conf:		https://pastebin.com/Vpqs0VxT
Server /etc/krb5.conf:		https://pastebin.com/1wnB6vum
Server /etc/ssh/sshd_config:	https://pastebin.com/WwdyQvF0

Guide followed for setup:	https://www.linuxtoday.com/blog/integrating-ldap-and-kerberos-part-one-kerberos.html
(at random times, the link is unavailable; use Google cache page if
needed)

Thank you for having read,

Rocky

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