>>>>> Ansgar Burchardt wrote:

> newszilla.xs4all.nl and news.arcor.de both seem to run Diablo.

> I found an interesting post about this [1]: apparently Diablo will
> forward a request by Message-Id to a different system, which likely has
> different article numbers.  If load balancing is used this might explain
> why I get sometimes different article numbers.

I see.  So, we should use only Message-IDs to refer to articles
for such news servers.

[...]

> If I post a followup to an article to a different group (e.g. a random
> *.test group) and then try to fetch the parent article it works without
> problems.  The problem only occurs if the parent was posted in the
> same group as the followup and Gnus tries to guess the article number.
> If Gnus behaved as in the first case, everything would be fine.

I studied how Gnus refers to parent articles, and realized that
Gnus uses an article number only when a parent article is in the
current group.  Could you try the patch attached below?

The new variable `nntp-xref-number-is-evil' controls whether Gnus
ignores article numbers in the Xref header.  This is a server
variable, so you can set it per server as follows:

(setq gnus-select-method
      '(nntp "newszilla"
	     (nntp-address "newszilla.xs4all.nl")
	     (nntp-xref-number-is-evil t)
	     ...
	     ))

Or you can simply use the following if your servers are all Diablo:

(setq nntp-xref-number-is-evil t)

Note that the number of a parent article in the current group
will be displayed as a negative number if you've configured
`gnus-summary-line-format', `gnus-summary-mode-line-format' or
`gnus-article-mode-line-format' to do so.

I verified the patch can be applied to both Gnus v5.11 of Emacs
22.1 and No Gnus v0.7.