I am running into this same issues when attempting ftp access for an astroneer server I just rented. Attempted access via filezilla, got the 503 login incorrect message. Tried changing the password via the web interface and get the home directory not found error. I am very interested if this problem is resolved.
I had a thought shortly after I wrote this, don't know if it will work for you, but I think my issue was because I had never actually used the server. Actually logging in and then out of the astroneer server fixed my issue. I think it just had not actually created the user directory. I am now able to change the ftp password and access it via filezilla.