OK, now I've got it. The problem was in a misconfigured proxy that
inserted a bogus Referer header (which is probably used by MediaWiki
to detect the need to redirect). So that for a request like that:
GET /wiki/V%E1clav_Havel HTTP/1.1
Host:
cs.wikipedia.org
Referer:
http://cs.wikipedia.org/wiki/V%E1clav_Havel
comes a "HTTP/1.0 200 OK" response with the "page does not exist yet"
contents.
After the proxy configuration was fixed, everything works fine.
Thanks for help,
-- [[cs:User:Mormegil | Petr Kadlec]]