When you log in you’re using the new API key as your password, and the username is set case sensitive
for ftp You will need to confirm the following settings:
Protocol: FTP - File Transfer Protocol
Host: ftp.marketplace.envato.com
Encryption: Use explicit FTP over TLS if available
Logon Type: Ask for password
User: <your Envato username here> (this is case-sensitive)
Password: <you will be prompted for the API key>
I use filezilla also, I ve get different errors like yours sometimes but these were all temporary errors. I wonder what is your max. upload speed provided by ISP? upload errors are happening more often at slow upload speeds,