VNC via SSH tunnel working or not, unkown causes

I have Ubuntu 20.04.3, in two computers PC1 and PC2. I can connect via VNC (remmina) between the two with no problem.

Now I try to use VNC via SSH tunnel.

1. Set the tunnel

    [PC1]$ ssh -L 5900:localhost:5900 [email protected]
    bind [127.0.0.1]:5900: Address already in use
    channel_setup_fwd_listener_tcpip: cannot listen to port: 5900
    Could not request local forwarding.
    Welcome to Ubuntu 20.04.3 LTS (GNU/Linux 5.11.0-41-generic x86_64)
    ...
   

2. In remmina, connect with Protocol Remmina VNC Plugin (the same as a non-tunneled connection) to localhost:5900. I am asked for the password set in Settings -> Sharing -> Screen Sharing. I enter that password, and I get Could not authenticate, attempting reconnection....

The same happens when trying PC2 -> PC1 (via SSH tunnel).

A. How can I fix this?

Now I try to use VNC via SSH tunnel. I can do that PC3 -> PC1, using a mingw64 prompt in PC3, with

1. In a mingw64 bash prompt,

   [PC3]$ ssh -L 5900:localhost:5900 [email protected]
   bind [127.0.0.1]:5900: Address already in use
   channel_setup_fwd_listener_tcpip: cannot listen to port: 5900
   Could not request local forwarding.
   Welcome to Ubuntu 20.04.3 LTS (GNU/Linux 5.11.0-41-generic x86_64)
   ...
   

2. In tigervnc, connect to localhost:5900. I am asked for the password set in Settings -> Sharing -> Screen Sharing. I enter that password, and I already have the VNC screen in PC3.

If I try to do the same with PC3 -> PC2, I get

1. In a mingw64 bash prompt,

   [PC3]$ ssh -L 5900:localhost:5900 [email protected]
   bind [127.0.0.1]:5900: Address already in use
   channel_setup_fwd_listener_tcpip: cannot listen to port: 5900
   Could not request local forwarding.
   Welcome to Ubuntu 20.04.3 LTS (GNU/Linux 5.11.0-41-generic x86_64)
   ...
   

2. In tigervnc, connect to localhost:5900. I am asked for the password set in Settings -> Sharing -> Screen Sharing. I enter that password, and I get Authentication failure.

B. How can I fix this?

The following info may be useful: In PC1 I have

$ gsettings list-recursively org.gnome.Vino
org.gnome.Vino prompt-enabled false
org.gnome.Vino require-encryption false
org.gnome.Vino use-alternative-port false
org.gnome.Vino disable-background false
org.gnome.Vino disable-xdamage false
org.gnome.Vino alternative-port uint16 5900
org.gnome.Vino icon-visibility 'client'
org.gnome.Vino authentication-methods ['vnc']
org.gnome.Vino network-interface ''
org.gnome.Vino notify-on-connect true
org.gnome.Vino mailto ''
org.gnome.Vino lock-screen-on-disconnect false
org.gnome.Vino use-upnp false
org.gnome.Vino vnc-password '<12-char string>'
org.gnome.Vino view-only false

In PC2 I have the same, except for

org.gnome.Vino vnc-password 'keyring'

And when I try to set the password in Settings -> Sharing -> Screen Sharing, I am limited to 8 characters, which is not the usual case I met.

I fixed the difference between PC1 and PC2, which led to

PC3 -> PC1 (SSH tunnel): OK
PC3 -> PC2 (SSH tunnel): Failed

In /etc/ssh/sshd_config in PC2, I had set before for security reasons AllowTcpForwarding no. Commenting that line, I could connect.

I then fixed the SSH tunnel PC1 -> PC2

1. Set the tunnel

    [PC1]$ ssh -L 5900:localhost:5900 [email protected]
    Welcome to Ubuntu 20.04.3 LTS (GNU/Linux 5.11.0-41-generic x86_64)
    ...
   

2. In remmina, connect with Protocol Remmina VNC Plugin (the same as a non-tunneled connection) to localhost:5902. I am asked for the password set in Settings -> Sharing -> Screen Sharing. I enter that password, and I get a remote desktop.

Same for PC2 -> PC1.