I think you typed an UTF-32 code in the multiplayer addressfield, so when it parses the string it throws this exception.
You can fix it, when you edit the starbound.config file in your rootfolder of starbound.
In this file is probably that line of code:
"title.connectionString" :
Delete the string after title.connection, when there is an UTF-32 Char in it, or otherweise search for an UTF-32 char in this file.