I have TurrisOS with OpenSSH + /bin/zsh as shell for root user. Worked just fine with previous versions. Since I updated to 3.11 I got:
% ssh router
root@router: Permission denied (publickey,keyboard-interactive).
Reason was:
2018-12-12 22:06:40 info sshd[13801]: User root not allowed because shell /bin/zsh does not exist
2018-12-12 22:06:40 info sshd[13801]: Connection closed by invalid user root 10.0.0.44 port 35218 [preauth]
Thus I issued ln -s /usr/bin/zsh /bin/zsh via web interface after which zsh revealed it was moved to /usr/bin.
This solved the issue for me. Of course I changed /etc/passwd and removed the symlink again after this.
I don’t blame TurrisOS for this, but this hint may help others. 