Example passive DCC that sets the port parameter to 0.
Keep in mind that passive DCC is a fairly new feature and the help file doesn't get updated regularly, so missing information about new features is common. You may consider it a bug, but that's just the way it is.
As for the router issue, take the time to search the forums and you'll find posts that deal with this and explanations as to WHY it fails. What basically happens (very quick summary) is that the router is changing the port from what mIRC expects, so as far as mIRC is concerned, it's trying to resume on a port that it shouldn't and so it fails. That is what happens in various NAT routers. Search the forum and you'll find more information.
Here is one short post for you about this issue.
Here is another.
Here is a possible fix for you and if you look up to Sojourn's post, you'll find probably the best explanation of why it happens.
