Like the other person said - that line of text contained at least one character that cannot be transported to the Server with the currently in use "Server Encoding".
Mudlet can negotiate with the server over which encoding to use but many Game Servers do not support the telnet option (42 "CHARSET") to do that so you may just have to find out what the Server uses and avoid anything outside that range slipping into your output. I did code in a tool to analyse (a line of) the on-screen text - but it has to be enabled - and after selecting the bit concerned if you hover over the contect menu option it will report what is there::