You really should talk to admin on your network or other mirc scripters who are experienced on your network.
Are you doing anything else on join in another script?
Maybe you should wait a moment before messaging them? It's a weird error message for a simple msg when someone joins. Target change too fast? Please wait almost 2 minutes? That just doesn't sound right. Please make sure you do not have another join event.
Options: You could write a queu that stores commands sent to the server and sends them out at a rate that won't trigger the flood detect, if that is a flood detect.
Maybe try a timer.
on 1:JOIN:#:{
if ($nick != $me) .timer 1 2 msg $nick Message Content
}