Boug using different OpenSSH versions
Oscaillaly I had problems with OpenSSH when using different versions, it would struck forever
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
I would just decrease MTU with ifconfig
ifconfig <interface> mtu 1200
It started bothering when wireguard interface started having this issue on OBSD-6.8-meta.
The server I'm connecting to, running OBSD-6.7 has OpenSSH_8.3 LibreSSL 3.1.1
I'm one subversion ahead on both.
Solution, from ServerFault
Chossing different MACs Algorithm than
# on /etc/ssh/ssh_config removed above algo MACs hmac-sha1,firstname.lastname@example.org
(so what's MTU? different MACs algo? no idea now /o\)