Hi there,
I have experienced the same problem, and have a little piece of the puzzle. As I looked at the server hosting my own private channel I saw the line
Unable to negotiate with XXX.XXX.XXX.XXX port 45072: no matching host key type found. Their offer: ssh-rsa [preauth]
which means that the guix pull command only uses a Hostkey using an algorithm that is not in the default configuration of the sshd HostKeyAlgorithms (as it is considered too weak for keys of a size <2048 bits?).
The workaround I am using is a line
HostKeyAlgorithms +ssh-rsa
in my server's sshd_config (and using a key of a size of 4096 bits).
Nevertheless, I would like to see guix pull using a host key with a different algorithm - or a larger variety of host keys.
Hoping that helps,
Cheers
Tim