Fatal error : Call to undefined function plural_name_from_cap_rs()

It sounds like you either had a file upload error or tried to access the site while upload was in progress. That function is defined before any access attempts to its functions.

But if you still think this is a source code error, you can rule it out by adding this right in role-scoper.php, right below the top comment block:

require_once( 'wp-cap-helper_cr.php' );

…and make sure you don’t have an old copy of wp-cap-helper_cr.php

After the update, all was running without any issue. But 30 min later, I had this error. I try to re-upload RS.

This error occurs when another plugin prevents Role Scoper from defining the pluggable WP function ‘set_current_user’. RS cannot coexist with another plugin that defines it or causes early execution of pluggable.php. However, the updated Role Scoper version 1.3.30 will fail gracefully, giving you an error message in wp-admin.

All is alright yet, after the re-upload of the plugin.

Moreover, the 1.3.30 version runs perfect.