Failed to add the host to the list of know hosts

This is the solution i needed.

sudo chmod 700 ~/.ssh/
sudo chmod 600 ~/.ssh/*
sudo chown -R ${USER} ~/.ssh/
sudo chgrp -R ${USER} ~/.ssh/

In your specific case, your known_hosts is a folder, so you need to remove it first.

For other people which experiencing similar issue, please check the right permission to your ~/ssh/known_hosts as it may be owned by different user (e.g. root). So you may try to run:

sudo chown -v $USER ~/.ssh/known_hosts

to fix it.