Sounds to me like a HHTT stratum implementation issue rather than anything else.
Thanks for your quick reply.

Sorry to bother you. Since I don't know anything about the protocol and about your code, let me know whether I understand correctly (so I will contact HHTT and I will be able to explain better the issue): that nonce isn't real, is it? Cgminer never found it and never sent it: it is faked by HHTT.
Otherwise I don't understand why cgminer didn't log it.
Thanks again.