Ok I found the problem...
I forgot just a thing:
ProxyPreserveHost On
By default it is OFF in apache mod_proxy and without it, xwiki has some
problem... I have been tracking this stupid problem for a long time :)
I found the solution while searching the mail archives... thks to the mail
archives :)
Pascal
On Mon, Jun 15, 2009 at 10:59 AM, Pascal Voitot <pascal.voitot.dev(a)gmail.com
  wrote: 
  Hello,
 I wonder whether this is a error in my servers configurations or this is a
 bug...
 I have an XWiki server running behind an Apache server and the link between
 them is performed using the simple mod_proxy with some pass such as:
 ProxyPass /xwiki/ 
http://localhost:8080/xwiki/
 It works perfectly except when I logout...
 I get redirected to an URL looking like:
 
http://localhost:8080/xwikihttp://mydomain/xwiki/bin/login/XWiki/XWikiLogin…
 Do you have an idea of the reason of this behavior?
 regards
 Pascal