WHMCS WP Integration Returns "WHMCS Integration: 401—Unauthorized".

Good morning,
I have installed WordPress in domain toot and WHMCS in /whmcs/ directory. So, their URLs are something like this..

WordPress: http://domain.com
WHMCS: http://domain.com/whmcs/

I have these settings at WHMCS WP Integration page:

Remote WHMCS host: http://domain.com/whmcs/
Default content page: http://domain.com/members/ (From the dropdown)

The WHMCS System URL (in WHMCS Settings): http://domain.com/whmcs/

However, I am getting a "WHMCS Integration: 401—Unauthorized" for all the short codes I have placed on /members/ page.

Can someone help me to fix this please?