Dopus FTP doesnt work, FileZilla does

I tries to connect to my website partner where I have lots of websites. It is possible to connect to these with ftp, but I'v never been able to do this with Dopus. Same connection goes with no problem when using FileZilla. FileZilla is free, but I paid to have full FTP with Dopus and I want i to function here as there are lots of other advantages I'm missing.

Why do I have this problem?

The FTP Doesn't Work FAQ has a list of things to try.