I heard about it a few times.
I think the problem is in your router/firewall not NATing the resume commands properly.
Check if there is a new firmware available for it.
I also think someone had the local info set up wrong. Check mIRC Options, local info for errors.
The resume use the same ports as an ordinary send.
What I meant is that there are buggy routers/firewalls out there. They mess up the request sent for a resume.