RSS Error: WP HTTP Error: couldn't connect to host

Hi guys/devs,
I am getting error:
RSS Error: WP HTTP Error: couldn’t connect to host on incoming links in Dashborad, also under WordPRess blog and news widgets.
To resolve the issue, I have:
1. Changed db user, updated new password, optimized table.
2. I have telnet my domain with port 80. (telnet worked)

However, the WordPress still showing error.
RSS Error: WP HTTP Error: couldn’t connect to host

Does anyone know how to fix it? Help. Please.