I just tested mIRC v7.45 and it worked fine for me. I set the port range in the mIRC Ports dialog, opened up that port range on my router, ran a second copy of mIRC on a another computer on a different IP address, and was able to send files from the first mIRC to the second mIRC without any issues. I also tested Passive DCC on the second mIRC and was able to send to the first mIRC without any issues.
Do you have UPnP support enabled in mIRC v7.45? If you disable that, does that help?