I just removed from the database all the tables added by Role Scoper, thata should be all the ones with the name ending with "_rs" (wp_groups_rs, wp_role_scope_rs ...)
by the way, a clean install is better. the point is that if you limit the permission of "Read" of a category/post and then you deactivate the plugin, firefox seems to keep this permission on, even if the plugin is not active. I could not mind this, even because I was working with Safari where all was going ok!
anyway, it's fixed. being more careful when using plugins is the best way, as always :)