ssh intermittent blocking connections

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I’m having intermittent problems with ssh in an “old” server running Ubuntu 12.04. I am getting this error message ssh_exchange_identification: Connection closed by remote host in most attempts. But from time to time I can connect by using ssh -vvv.
This server (ventus) is part of a cluster whose nodes (ventus2-5) were configured with ssh-keygen to ssh without asking for a password, it was running fine until this week. Now I can ssh from the main node (ventus) to any of ventus2-5 but it is not possible to ssh from the other nodes to ventus.
This is the output of ssh -v from node 5 (ventus5) to the main node (ventus)
[email protected]:~$ ssh -v ventus
OpenSSH_5.9p1 Debian-5ubuntu1.1, OpenSSL 1.0.1 14 Mar 2012
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug1: Connecting to ventus [192.168.1.1] port 22.
debug1: Connection established.
debug1: identity file /home/meteo/.ssh/id_rsa type 1
debug1: Checking blacklist file /usr/share/ssh/blacklist.RSA-2048
debug1: Checking blacklist file /etc/ssh/blacklist.RSA-2048
debug1: identity file /home/meteo/.ssh/id_rsa-cert type -1
debug1: identity file /home/meteo/.ssh/id_dsa type -1
debug1: identity file /home/meteo/.ssh/id_dsa-cert type -1
debug1: identity file /home/meteo/.ssh/id_ecdsa type -1
debug1: identity file /home/meteo/.ssh/id_ecdsa-cert type -1
ssh_exchange_identification: Connection closed by remote host

trying ssh -vvv I get the same error most times but from time to time is it possible to connect, both from cluster nodes or external IP (PINAR host trying to connect to ventus)
[email protected]:~$ ssh -vvv ventus
OpenSSH_8.2p1 Ubuntu-4ubuntu0.1, OpenSSL 1.1.1f 31 Mar 2020
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: include /etc/ssh/ssh_config.d/*.conf matched no files
debug1: /etc/ssh/ssh_config line 21: Applying options for *
debug2: resolving “ventus” port 22
debug2: ssh_connect_direct
debug1: Connecting to ventus [XXX.XXX.XXX.XXX] port 22.
debug1: Connection established.
debug1: identity file /home/meteo/.ssh/id_rsa type -1
debug1: identity file /home/meteo/.ssh/id_rsa-cert type -1
debug1: identity file /home/meteo/.ssh/id_dsa type -1
debug1: identity file /home/meteo/.ssh/id_dsa-cert type -1
debug1: identity file /home/meteo/.ssh/id_ecdsa type -1
debug1: identity file /home/meteo/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/meteo/.ssh/id_ecdsa_sk type -1
debug1: identity file /home/meteo/.ssh/id_ecdsa_sk-cert type -1
debug1: identity file /home/meteo/.ssh/id_ed25519 type -1
debug1: identity file /home/meteo/.ssh/id_ed25519-cert type -1
debug1: identity file /home/meteo/.ssh/id_ed25519_sk type -1
debug1: identity file /home/meteo/.ssh/id_ed25519_sk-cert type -1
debug1: identity file /home/meteo/.ssh/id_xmss type -1
debug1: identity file /home/meteo/.ssh/id_xmss-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.2p1 Ubuntu-4ubuntu0.1
kex_exchange_identification: Connection closed by remote host

from iptables -nL I found this
Chain sshguard (1 references)

DROP all — 192.168.1.1 0.0.0.0/0

Have tried different options for iptables and removed this last line but the problem persists.
Any idea will be very welcome, thanks in advance

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Marisa

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