After clearing the cookies (not clearing the cache!) and refreshing the browser
it works.
I think the template chooser sets a cookie and only the template
stored in the cookie will displayed in the frontend.To fix that a solution could be that mambo must ignore the
mos_user_template
statement in the cookie if the templatechooser module is
disabled!
The mos_user_template statement in the cookie should only used if the template chooser module is activated.