Can't login to WPMU DEV through wordpress

I have a VPS running Ubuntu and Nginx and php5-fpm to host a wordpress multisite , I have installed WPMU Dev plugin, but I cannot login to it from the wordpress admin console.
In the network managemenet, I click on WPMU DEV and dashboard, there I enter the credential but I receive the following error :

"Is php on my server properly configured to be able to contact https://premium.wpmudev.org/wdp-un.php with a GET HTTP request via fsockopen or CURL?"

I have checked and confirm that CURL is installed on the php on the server.
I also contact with my VPS provider and they answered that there is no limitation from their side.
I also disabled the ufw firewall on the ubuntu to test the function but nothing changed.
Please kindly help to resolve the issue.