It seems that other users are already connected with your hostname and therefore, to prevent clone abuse, no more connections are being allowed. If you are sharing a connection with a school or work place etc, then it might be because too many people are already using that IRC network already. Check
Undernet's homepage to find out their rules on clones and how many connections they will allow.
The reason you can connect to other networks is because:
1) Other networks probably don't have as many users connecting to their network with that hostname, and
2) They may not have such stringent rules on how many clones may connect
It could also be a server-only regulation, so try connecting one of the server pools (group of servers) that are related to Undernet. For example, try connecting to
irc.undernet.org or if you're in the US,
us.undernet.org or for the UK
uk.undernet.org.
Good luck!
Regards,
