GNUS generates `Date:' headers without time zones. C-News's `inews' doesn't replace it with a valid header, but will generate it if not already there. If it is invalid, the article will not be forwarded properly. Quick fix:
(defun gnus-inews-date () nil)
I think this is fixed in GNUS 3.14.1 if you set gnus-news-system {?} to Cnews.