SSH – Asked for Passphrase for Passphrase-less Private Key on Virtual Machine

I'm having some issues SSH'ing to a server when logged into a particular VM.

From my local PC, I can log in to the server just fine using the command ssh -i .ssh/[my private key] username@domain.

If I then SSH into another unrelated server which also has the same private key and run the above command it will also log in just fine in to the server.

But, when I SSH into a particular OpenStack VM (again has the same private key) on the unrelated server and then SSH in to the server from there I start getting prompted for a passphrase. But a passphrase hasn't been set for the key so inevitably I get a "Permission denied (publickey)." error.

To double check, I removed the private key from the VM and scp'd the private key from my PC into it again and tried the same command I still get prompted for a passphrase.

In other words:

1. PC – SSH – Server1 = Works fine
2. PC – SSH – Server2 – SSH – Server1 =
   Works fine
3. PC – SSH – Server2 – SSH – Virtual Machine – SSH – Server 1 =
   Asks for a passphrase

What could be the reason behind this? It's definitely the right private key I'm using and it has the appropriate permissions.

Edit – Output of ssh -v -i .ssh/[my private key] username@domain as requested

OpenSSH_7.4p1, OpenSSL 1.0.2k-fips  26 Jan 2017
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 58: Applying options for *
debug1: Connecting to xxx.xxx.xxx.xxx [xxx.xxx.xxx.xxx] port 22.
debug1: Connection established.
debug1: key_load_public: No such file or directory
debug1: identity file /.ssh/[my private key].ppk type -1
debug1: key_load_public: No such file or directory
debug1: identity file /.ssh/[my private key].ppk-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_7.4
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.2p2 Ubuntu-4ubuntu2.4
debug1: match: OpenSSH_7.2p2 Ubuntu-4ubuntu2.4 pat OpenSSH* compat 0x04000000
debug1: Authenticating to xxx.xxx.xxx.xxx:22 as '[my username]'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: curve25519-sha256@libssh.org
debug1: kex: host key algorithm: ecdsa-sha2-nistp256
debug1: kex: server->client cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
debug1: kex: client->server cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
debug1: kex: curve25519-sha256@libssh.org need=64 dh_need=64
debug1: kex: curve25519-sha256@libssh.org need=64 dh_need=64
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ecdsa-sha2-nistp256 SHA256:ZiugTjR5fM0E3evOwHoePFKspDQChA0Ab6L0q88KP/g
debug1: Host 'xxx.xxx.xxx.xxx' is known and matches the ECDSA host key.
debug1: Found key in /home/centos/.ssh/known_hosts:2
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info: server-sig-algs=<rsa-sha2-256,rsa-sha2-512>
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Trying private key: /.ssh/[my private key].ppk
Enter passphrase for key '/.ssh/[my private key].ppk':
Enter passphrase for key '/.ssh/[my private key].ppk':
Enter passphrase for key '/.ssh/[my private key].ppk':
debug1: No more authentication methods to try.
Permission denied (publickey).