Bad display of registration page

Hi All

I install joomla platforms for quite some time and I've never encountered this kind of problem (see my attachment)

And when i replace CB to the native module (Connexion) i've an error 500

If you want to test, this is the url (click on inscription or créer un compte)
altidia.biz/mygestion25/

Configuration :
Joomla! 2.5.7
CB 1.8.1
MySQL 5.1.49-3
PHP 5.2.17

I already erase all and load a new instance (DB+site)

If you understand, thanks for your help
Thierry

And the answer is : You put "no" in the SEO/SEF options

Don't ask me why, i don't know. On 3 sites, 2 are in this case the third works fine despite that the SEF is to "yes".
And this is the same joomla's pack that i used to instal the all

All the best
Thierry