Notepad++ NppFTP plugin fails to connect via SFTP

The solution for me was to use puttygen to export a .ppk version of my private key to OpenSSH as follows:

1. Open PuttyGen
2. Menu: Conversion -> Export OpenSSH key
3. Save your new key (don’t forget to make it clear it is openssh)

This was weird seeing as my .ppk was converted from a private key generated by OpenSSH itself.

I think it has to do with the fact that my original key starts with

---- BEGIN SSH2 ENCRYPTED PRIVATE KEY ----

whereas the exported key starts with

-----BEGIN RSA PRIVATE KEY-----