On http://phd.martin-krafft.net/wiki/tag/factors/, the top-level factors entry is not linked to the corresponding page. Looking at http://phd.martin-krafft.net/wiki/tag/factors/language/, this must be because the page name is the same as the entry name, and ikiwiki probably doesn't take the complete path of subpages into account.
I can confirm that most if the issues are fixed, but map still includes and links to pages that do not match the pagespec. The list includes entries like tag/factors/contribute, but that page does not link/is not tagged with any factors* tag. I have put a snapshot of the site as it was when I saw this bug at http://scratch.madduck.net/web__phd.martin-krafft.net__map-bug-1.tgz and can return to the state at any time, but I needed to move on now...
--madduck
That's a different issue. 
This is really subtle and tricky. It's doing this because it thinks that tag/factors/contribute does link to a page matching "tag/factors/*". That page? tag/factors/contribute/discussion!
Now, tag/factors/contribute/discussion doesn't exist yet in your wiki, but there is a "?Discussion" pseudo-link, and that's good enough for ikiwiki.
So, you could work around this annoying behavior with !link(tag/factors/*/Discussion)
BTW, the testpagespec plugin is useful in debugging these kind of things.
--Joey