SFTP Password prompt

[pdubbelman]

When trying to connect to a client's SFTP server using a key file, I'm prompted for a password (see attachment).
I push files to the client's SFTP server, so I have the private key, and the client has the public key

I'm quite sure the key files were generated without a passphrase (and if it was generated with a passphrase, then I don't have that passphrase )

I have a few questions about this:
- Is this password prompt for the passphrase of the key file, or is it for the SFTP server?
- If it is for the key file, does the prompt only appear when the key file was generated with a passphrase, or does it also appear when the key file doesn't have a passphrase?
- Is this password prompt initiated by FileZilla before it even tries to setup the connection, or is it initiated by the client's SFTP server that is somehow configured to always require a passphrase?

Thanks for your help!

[botg]

The prompt is clearly for the password to the key file. It is only shown for key files that are protected by a password, at the point when the key file is about to be used for authentication.