Major Description
On the configuration page for the server, http://cwiki.apache.org/FTPSERVER/configuration.html, no mention is made for the max-logins attribute. By default, this is set to "10" (a pretty low number). The documentation should show configuration of this attribute.
For example,
<server xmlns="http://mina.apache.org/ftpserver/spring/v1"
xmlns:beans="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
http://mina.apache.org/ftpserver/spring/v1 http://mina.apache.org/ftpserver/ftpserver-1.0.xsd"
id="Server1" max-logins="200">
Failure to configure this may result in mysterious, "421 Maximum login limit has been reached."