I have noticed the problem and took the liberty of pinpointing the cause. It seems that when you are sent a dcc send or chat request from the same ip on the same port 2 times in a row, it fails. If you give it a minute, it will actually work 2 times in a row, but for some reason not one right after the other.

A lot of people won't notice it right away because most senders use different ports each time, but not all! many of us are behind routers and dedicated certain ports for dcc.

Here is a way to test the bug.

Code:
sendchat { .raw privmsg $$1 $+(:,$chr(1),DCC CHAT CHAT $longip($ip) $$2,$chr(1)) }


try //sendchat $me 12345 several times and see it fail.
then try changing ports each time and doing it over and over. It works. weird.