Does
http://www.dokuwiki.org run the latest stable release of DokuWiki (2011-05-25a “Rincewind”)?
I did a bit more testing, and in the Zotero wiki installation
"/support/2.1#upgrading_from_zotero_1.0" resolves to "http://www.zotero.org/support/2.1#upgrading_from_zotero_1.0"
"/2.1#upgrading_from_zotero_1.0" resolves to "http://www.zotero.org/2.1#upgrading_from_zotero_1.0"
"2.1#upgrading_from_zotero_1.0" resolves to "http://www.zotero.org/support/playground/2.1#upgrading_from_zotero_10"
so the problem only seems to present itself when a link contains a slash. I'm not able to reproduce this in the playground of
http://www.dokuwiki.org, though, so it's probably fixed already, as you said.