After looking on wp-admin/user-new.php, I'm finding that there is a check for 'promote_user' (without final "s") capability on line 60.
if ( ! current_user_can('promote_user', $user_details->ID) )
Is this capability is an error (plural ending forgotten) or is it wanted ?
I make the modification on my installation and it works better for me...
Sorry for my English and thanks for your future answers,
PS: the only one mention of promote_user that I found is on this ticket