Hello Daniel,
I have never tested on windows. One possible scenario that might result in an error like this is if Clozure is using a stream encoding/locale where it helpfully inserts carriage returns after newlines in stream write operations. I'm not familiar enough to with Clozure to know what to look for, though. You could try capturing packets from a Postmodern client running on a Unix with those sent for the same commands by a Windows client, that'll probably pinpoint the problem for you.
Best, Marijn