The reason for this is that the server is not sending the expected IRC numeric 353 format. The RFC states that numeric 353 needs to use a colon : to indicate the start of the nickname list.
In addition, if I recall correctly, one of the reasons mIRC wants a colon is that, long ago, some ircds used a slightly different 353 format, and the only way to know where the nickname list began was to look for a colon.
You would need to contact the developer to ask them to prefix the nick list with a colon, per the RFC.
