FTP uses two connections, one for data and one for commands. The PassivePorts configuration is the range of ports to be used for passive FTP data transfers.
The Virtualbox network configuration allows you to enter single ports to forward, but not ranges. So, in order for passive-mode FTP to work, the passive ports must be forwarded individually.
If you don't expect more than 5 simultaneous data transfers (and clients like Filezilla will open 1 data connection per file and per directory listing) then change the server configuration to use a range of 5 ports (e.g. 1025-1030) and forward those five ports in the Virtualbox configuration. For 10 connections use a 10 port range, etc.