I already posted an information of this bug in the general moddle forum.
http://moodle.org/mod/forum/user.php?id=103936&course=5
The problem may happen when you install the lastest version of DFWiki with PHP >= 5.1.1 .
This has something to do with the outdated library markdown.php . To solve the problem, just get the latest version of markdown.php .
(well, it worked for me !)
Best regards,
Thierry Nabeth
http://www.calt.insead.edu/