my first question here, so pls bare with me.

I have a weird and complex problem where i dont know where to start troubleshooting.

During setup of my “backup-nas” i suddenly lose putty ssh connection while typing in some random command.

No Certificates configured on either end.

The only way to recover this is by

1. Remove ssh host keys from putty in regedit on the client
2. Remove ssh favorite entry from putty in regedit on the client
3. Then restart the windows client
4. Then try several reconnects until it finally asks me if i want to accept the connection, then it works for a while for lets say an hour until the problem comes back.

Neither Putty nor Windows SSH would work then

Weirdly

• I do run 3 other Ubuntu servers in my LAN which dont have that porblem with my client
• Any other of my ubuntu servers is able to ssh to the new NAS system without any of such problems. So i know that ssh on the server end is still alive. Now the thing is that my windows client doesnt have any sort of problems with the 3 other Servers in my LAN Environment.
• The NAS itself doesnt log any weird thing during boot, cold start or shutdown, dmesg.
• Auth.log and journalctl -u ssh dont even log that attempt to connect when it fails.

Only related post i found is this:
https://askubuntu.com/questions/603911/unable-to-connect-to-my-ubuntu-machine-using-putty-remotely
No luck

Thanks in advance

What i have done so far

1.) I updated windows 10 to windows 11 on the client without luck

• Random suggestion from the web, but that update was pending anyway

2.) I completely changed HW on that NAS inlcuding Motherboard, Networkcard, Powersupply, CPU and Memory

• Because i ran into different problems with initial 24.04 install on various ends not ssh related.
• New HW however didnt fix the problems i had with 24.04

3.) Reinstalled the new HW NAS completely from scratch with 22.04 instead of 24.04

• Because any other of my servers runs 22.04 and it appeared that 22.04 on that HW is working just fine now

4.) Defeating Windows Firewall

• I had in mind it may have any effect – no luck

Netplan on 192.168.1.13

network:
  ethernets:
    enp2s0:
      dhcp4: no
      addresses: [192.168.1.13/24]
      routes:
        - to: default
          via: 192.168.1.1
      nameservers:
        addresses: [8.8.8.8,1.1.1.1]
  version: 2
  renderer: networkd

NMAP from 192.168.1.11

Nmap scan report for 192.168.1.13
Host is up (0.00013s latency).
Not shown: 997 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
139/tcp open  netbios-ssn
445/tcp open  microsoft-ds

ip a on 192.168.1.13

2: enp2s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 00:90:27:a4:8b:ac brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.13/24 brd 192.168.1.255 scope global enp2s0
       valid_lft forever preferred_lft forever
    inet6 fe80::290:27ff:fea4:8bac/64 scope link
       valid_lft forever preferred_lft forever

iptables -S on 192.168.1.13

joe@backup:~$ sudo iptables -S
-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT

sshd_config

Include /etc/ssh/sshd_config.d/*.conf
Port 22
AddressFamily any
KbdInteractiveAuthentication no
UsePAM yes
X11Forwarding no
PrintMotd no
AcceptEnv LANG LC_*
Subsystem       sftp    /usr/lib/openssh/sftp-server