This sounds like a response from the network, in which case your mIRC registration is irrelevant to the problem.
Can you copy & paste the exact message that you're getting including all details?
Another possibility might be that you're not sending the information to the network in sufficient time, which generally means you have to specify the required password for the network in the network setup.
This area can be located by
1) pressing Alt + O (Options)
2) Select the connect category
3) Select Servers
4) Click Add or select the network & server in the drop down boxes and click Edit
5) Fill in the required information
6) Click OK
7) Click OK
Without more details, this is the best I can do.