Kept getting an "Invalid private key file" error when trying to connect to Siteground using an SSH private key file over SFTP on port 18765. The keyfile worked perfectly on Filezilla, so was quite confusing. Turns out it's the format of the private key file needs to be updated to OpenSSH for it to work with Notepad++.
eg. the log was :
Connecting
[SFTP] Host key accepted
[SFTP] Invalid private key file or incorrect passphrase
[SFTP] Error during authentication: Access denied for 'none'. Authentication that can continue: publickey
Therefore
Open PuttyGen.
Load the key.
Go to the Menu -> Conversion -> Export OpenSSH key
Save your new OpenSSH key
Use that in Notepad++ instead. Hey presto! it works!
Original solution found here:
https://stackoverflow.com/questions/8939684/notepad-nppftp-plugin-fails-to-connect-via-sftp