What error do you get? Are you using FTP, FTPS or SFTP or SCP?

Did you try to setup cloudwatt endpoint in URL?

It might be because file is locked as it is opened.

Yes, name + size is an option in Sync criteria drop down list in Menu -> Options

Sorry for the delay, did you get "Invalid credentials" error? If not then it means connection succeeded but file list failed possibly due to date format error returned by the SFTP server. You can see what AndFTP does in the bottom small window, it reports CONNECT, LOGIN, LS ... We need to know what ...

It might be because of date format issue, does it work without SSH key? If it's a date format issue then it won't.

Set criteria to name + size + newest date. The name + size + date fails on many device due to Android OS issue to modify file timestamp without root access.

Did you try to transfer some files from remote folder to local?
AndSMB is a tool to transfer files remote<->Local.

Yes, you can if you have rooted phone.

Try again with Enabled(Full)