Secure iPrint, Netware OES, Win XP SP2

We are seeing this error randomly on nearly all unmanaged Win XP clients
on Firefox and IE (latest versions) when downloading printers from the
iPrint webpage.

Error Group:
Error code 6
Error message: An unknown error occurred

If you cancel the error (which happens after authentication) and try to
download the printer again all works fine. There appears to be no
pattern to the error appearing (ie not specific driver related).

Does anyone know what causes this error, and how to eradicate it as it
confuses our users.