Bug#747096: openssh-client: key negotiation hangs for mtu 1500 and default Ciphers options

Package: openssh-client
Version: 1:8.4p1-5+deb11u1
Followup-For: Bug #747096
X-Debbugs-Cc: [email protected]

Dear Maintainer,

* What led up to the situation?
Some network changes not under my control

* What exactly did you do (or not do) that was effective (or
     ineffective)?
Standard connetion command ssh <user>@<remote_machine>

* What was the outcome of this action?
Hangs on key negotiation
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY

* What outcome did you expect instead?
To be aske for password as ussually

I tested this situation in problematic network from this computers:
1) Fresh Debian 11 install I am reporting bug from
2) My other Ubuntu 20.04 notebook and Ubuntu 20.04 server have the same problem
3) Windows 10 putty, Rocky Linux 9 work without problem

MS

-- System Information:
Debian Release: 11.4
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable-security'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 5.10.0-16-amd64 (SMP w/1 CPU thread)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages openssh-client depends on:
ii  adduser           3.118
ii  dpkg              1.20.11
ii  libc6             2.31-13+deb11u3
ii  libedit2          3.1-20191231-2+b1
ii  libfido2-1        1.6.0-2
ii  libgssapi-krb5-2  1.18.3-6+deb11u1
ii  libselinux1       3.1-3
ii  libssl1.1         1.1.1n-0+deb11u3
ii  passwd            1:4.8.1-1
ii  zlib1g            1:1.2.11.dfsg-2+deb11u1

Versions of packages openssh-client recommends:
ii  xauth  1:1.1-1

Versions of packages openssh-client suggests:
pn  keychain      <none>
pn  libpam-ssh    <none>
pn  monkeysphere  <none>
pn  ssh-askpass   <none>

-- no debconf information