Hmm, that’s a good one. Since 6.3.0_14 the XDMCP/Xnest connection will check if the font server is available first if one is configured and then connect. Not fonts, no connection.
The fact that it fails says it held the font server config, is it possible to do a ethereal capture of when it connects to the font server to see what happens? Failing that its one for Wyse support sorry.