It happens quite often that I cannot send files to other persons (or they can't receive). We get the error: connection refused. Sometimes, I can resume to send after switching passive DCC from on to off or vice versa. What is going on here? Why are so many people having trouble with DCC send and why does it SOMETIMES work after switching the passive DCC setting? Btw: I don't have a router and I use mirc 6.17.

For DCC's to work a direct connection must be established between the two IRC clients involved (hence "Direct Client-to-Client"). Many firewalls, routers and modems block incoming connections.

By default the onus is on the initiator of the DCC to have a setup which allows incoming connections to reach their client, but with passive DCC this is reversed (the receiver, not the initiator, must have a correct setup).

To know how to fix your end you must first determine how you connect to the internet (e.g. which models of modems/routers you use) and what, if any, firewall/security software you are running.

For modem/router configuration visit portforward.com
For the Windows XP Firewall (pre-SP2) follow this tutorial
For the Windows XP Firewall (SP2) follow this tutorial