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Subject: No supported authentication methods available (server sent public key)

Filezilla is able to use ssh-agent if it is running. It should not be necessary to import any .ppk format keys; otherwise, unenecrypted keys can be created with puttgen. The client should be set to "Interactive", for key-based login.

This error appears similar to "(permission denied, public key)" in OpenSSH. Make sure that the account lists the contents of id_rsa.pub in the relevant authorized_keys file. If the user can't log in via the command line with ssh, then Filezilla certainly won't work, so you might as well debug that first.