By "relative" I mean the last few years. Googling shows that the D-Link DI-524 supports UPnP, which means it's relatively new enough. UPnP is a much simpler method of configuring DCC, and I'd recommend it if you're having trouble. You do need 7.1 for this, though.
Upgrading mIRC should generally not affect the script-- if it does, the issues should be minor. I can't guarantee any of this, but it's certainly worth a shot. 7.1 adds plenty of fixes, especially if you're on win7, and possibly has some DCC related changes that may have to do with your issue. It's really hard for any of us here to support a version of mIRC that is almost 4 years old.
If 7.1 does not work, you should at least be upgrading to 6.35, which should be at least 99.999% compatible with your script. This would rule out any dcc issues that had been fixed in recent years.