OpenSSH 8.3p1 introduces the need of having a public SSH key otherwise is gonna show the warning
load pubkey "/root/.ssh/id_rsa": invalid format
# ssh -V
OpenSSH_8.3p1, OpenSSL 1.1.1g 21 Apr 2020
To supress this message we can put in place our SSH public key or generate it from our private key with:
# ssh-keygen -y -f /root/.ssh/id_rsa > /root/.ssh/id_rsa.pub
As pointed out in https://bbs.archlinux.org/viewtopic.php?id=256210 there is already a commit ready to fix this in the future.
Maybe you're using UTF8-bom. You can use
sed -i 's/^\xEF\xBB\xBF//g' /root/.ssh/id_rsa
to remove bom in the rsa file.`