Well there's your problem.
Checking "server connections" under port range makes mIRC use the ports in said port range for its local port. Obviously the next port in the range was in use when you tried to connect. Do you have multiple copies of mIRC running? Alternately, did you connect to more servers than your port range can support? My port range is 1024-1030. With the "Server Connections" setting on, I am only able to connect to 6 servers, and that assumes I have no DCCs active.
I can reproduce this if the port is in use, as expected. However I can't reproduce this with a single mIRC and no applications using that port range. It only happened once when I reconnected quickly, but likely the TCP connection was not yet fully closed in that attempt. It resumed functioning in the very next attempt. If that's the behaviour you're seeing for extremely fast reconnects, that's somewhat normal, but it shouldn't happen more than once in a row.
In short, don't check "Server Connections" unless you know what you're doing. It works as intended, but it probably is not what you want.
