cannot ssh edit a file

[Luca Ferrari]

Hi all,
I'm using emacs 26.3 on ubuntu linux, and I cannot tramp via ssh into
a machine to edit files.
What happens is that once I digit /ssh:luca@remotehost: Emacs freezes.
I've ssh keys in place, and I cannot connect with a terminal and ssh
regularly. I've tried changing the shell of the user, since the remote
user was using zsh and I've read that some particular prompts can
confuse tramp, but nothing changed.
I've tried the same using M-x find-file, but it freezes again.
I've noted that trying to connect with a user that does not have ssh
keys prompts me immediatly for the password, but once the password is
entered, emacs freezes.

Any idea about how to investigate and fix the problem?

Thanks,
Luca