mIRC does not use the specified DCC-Ports.
I set mIRC to use all ports from 2121 to 2122 (First: 2121 - Last: 2122). Sometimes DCC Send did not work (although my router forwareds 2121 and 2122 to my PC). That's the reason I enabled /debug before trying to send the file.
mIRC often wanted to use Port 2123 then.
I think it's a bug isn't it? In my opinion 2123 is not between 2121 and 2122