Filezilla Unknown Certificate
It appears the latest update of Filezilla 3.10.0 - is forcing FTP over TLS rather than plain FTP.
When quick connecting you may be prompted with an 'Unknown Certificate' dialog box as per screenshot below -
It is a self signed certificate and is secure and encrypted.
Simply accept the certificate by pressing 'OK'
Then you will need to navigate to the following > Edit > Settings > FTP
Ensure Transfer Mode is set to 'Passive' (See screenshot below)
However, if you are connecting to a Cloud VM then it should be set to 'Active'
Related Links -