Hey Ivan
I don't have the response may be you are sending super big documents and you are hitting the default HTTP size limit? But I'd not expect such a message in that case.
Or may be there is a malformed document or id which is causing the response not to be parsed correctly?
May be you can get more information by adding the httpclient to debug mode?
Anyway and not really related, may I suggest that you look at the BulkProcessor class instead?