Linux – rsync: Permission denied (publickey) with SSH

I have a server on which I connect in SSH with my key files such as id_rsa.pub. 
I am using Debian on client and server side.

I followed a tutorial to disable root authentication and password use for more security (by configuring the /etc/ssh/sshd/sshd_config file on the server).

Until now, I could easily use rsync to synchronize my files from my computer to the server.

A few days ago, I used a cron job job and for that I had to reset my root password on the server side.

Since then, it has been impossible for me to use rsync in SSH;
I get the following message:

nextcloud@192.168.0.20: Permission denied (publickey).
rsync: connection unexpectedly closed (0 bytes received so far)[sender]
rsync error: unexplained error (code 255) at io.c(235)[sender=3.1.3]

There are some similar topics, but I think my case is a bit different because I think the problem is due to resetting my password on the server. I have also asked a question on the Ask Ubuntu site,
but I think maybe people here are more expert in permission issues.

I have no idea how to fix the problem (I manage a little bit in terminal commands, but I'm not a computer expert). Could you help me, please?

For Your Information only, please find the tutorial tweaking the /etc/ssh/sshd_config to not use password:

#Uncomment or add the following line. 
#This allows the server to give its DSA footprint in case of an ssh connection.
HostKey /etc/ssh/ssh/ssh_host_dsa_key

#Then set the next parameter to 20s (for example). 
#This is the time during which a connection without being logged in will be opened. 
#If we had kept the good old password technique, leave 2 or 3 minutes to type it, it's not too much. 
#But since we're using the key now, we'll be logged in immediately. #So we can really reduce the thing and put it down to 20 seconds for example.
LoginGraceTime 20s

#this is the maximum number of attempts before being thrown by the server.... 
#Since with the key, no possible error, you can put it to 1 possible test.
MaxAuthTries 1

#Then, we will tell the SSH server where the keys are and tell it that we will use them as an authentication method
PubkeyAuthentication yes
AuthorizedKeysFile.ssh/authorized_keys

#And of course, we'll disable all other authentication methods
RSAAuthentication no.
UsePAM no
KerberosAuthentication no
GSSAPIA Authentication no.
PasswordAuthentication no

#Then, we will tell that we only allow users of the sshusers group (for more security)
AllowGroups sshusers

#The MaxStartups setting indicates the number of un-authenticated ssh connections you can launch at the same time. 
#2 is more than enough, knowing that with the keys, it's instantaneous.
MaxStartups 2