When viewing a wiki page (other than the front page), the header contains a (broken) link to the front page.
The link is broken because no wiki ID is specified in the URL.

To correct this, edit mod/wiki/view.php to replace moodleID with cm->id, as shown below

diff view.php.orig view.php
279c279
< (($ewiki_title!=$wiki->name) ? " > <a href=\"view.php?id=$moodleID\">".format_string($wiki>name,true)."</a>":"").
—
> (($ewiki_title!=$wiki->name) ? " > <a href=\"view.php?id=$cm>id\">".format_string($wiki->name,true)."</a>":"").