I may have the same issue.
Whenever I start the UnrealIRCd service without a network connection (network cable unplugged) and then I connect to a network the server doesn't respond to NICK requests from a client running on the same or different machine. If i restart the service after it is connected to a network then I am able to connect to the server.
This problem occurs with UnrealIRCd 4.0.11 and 4.0.12 but not in 3.2.8.1
Here is what i see in wireshark
Server: *** Looking up your hostname...
Client: *** Request USER [username]
Server: *** Couldn't resolve your hostname; using your IP address instead
Client: Request NICK [nickname]
--no response--
[Then I Connect the Computer running UnrealIRCd to a local network (no internet)]
Restart the service
Server: *** Looking up your hostname...
Client: *** Request USER [username]
Server: *** Couldn't resolve your hostname; using your IP address instead
Client: Request NICK [nickname]
Server: Ping
Client: Pong
............