I too am having this problem. I can only send or recieve one DCC at a time, however the specifics are a little different for me.
When I do a fresh install of winXP, mirc6.16 works great. I am running sygate pro firewall and use another machine on my lan for virus scanning ( NO antivirus on the machine running mirc and all machines are behind a router, and DCC is fully functional as in I can send and recieve multiple DCC's at the same time). For some reason, after about a month I can only send or recieve one DCC at a time.
I am an experienced mirc user and I know that all my options are correctly set, ignores are off and ports are set and holes are punched into my firewall and router. I am totally perplexed at to why my DCC's do not fully work.
I generally only install programs like MSoffice and maintainance utilities for the system, its not for gaming, photo/digital editing or anything other than email, surfing and scripting. ( When I am scripting I do not use my personal client that I normally use to chat) Does anyone have an idea as to a solution? Im kinda getting tired of reinstalling winXP to get functionality out of mirc, and I do not necessarily want to revert to an archaic version of mirc.