Well, I changed the Assignee property from UserList to DBList.
It's not so easy to find documentation on this subject.
This page helped me a little
http://dev.xwiki.org/xwiki/bin/view/Drafts/DatabaseListProperties
2011/12/1 Maxime Sinclair <maxime.sinclair(a)gmail.com>om>:
  Hi,
 I try to use the Todo Application
 (
http://extensions.xwiki.org/xwiki/bin/view/Extension/Todo+Application)
 in a farm context with all the users defined in the master wiki.
 The only problem is the Assignee property defined as a "List of
 Users". In a subwiki of the farm, the only defined users are
 XWiki.Admin and XWikiGuest.
 It would be fine if the list of users can be filled from the
 XWiki.AllGroup of the subwiki. But I don't know how to do this. Could
 you put me on the right direction ? May be you have a better idea...
 I hope my question is clear.
 Maxime