I restored the DB but with no difference.
Also removed all SEO files in extraXXXX folders or restored with the original ones from the ZEncart installation folder.

I am prepared to reinstall Zencart again but I would like to know exactly where to put a file from the Ultimate SEOurl module.
In the folder "_zen_cart_folder_English" I see a folder admin , a folder includes and a "htaccess_sample" file.
So I conclude this htaccess_sample (which also has to be renamed to .htaccess) SHOULD be installed in the main directory.