Impossible to register a new user

[lion at igr.fr]

Hello,
After installation of xwiki (+Tomcat 5 + Postgresql 8.2).
I would like to realize a user registration (inscription/register) 
but the proposed page is the same that for the login (identification/log - in).
and so i can't add user, when I try, the system answer me:
"Wrong user name"

In tomcat logs i find this error:
17:08:57,046 
ERROR http-8080-Processor23 
http://localhost:8080/xwiki/bin/register/XWiki/Register 
SimpleLog4JLogSystem:logVelocityMessage:154 
- Left side ($objuser.getProperty("usertype").value) of '==' operation has null value. 
If a reference, it may not be in the context. 
Operation not possible.  
[line 10, column 66]

can you help me please?
Thanks,
François Lion