Problem Desc - Cannot reach WP servers. Cannot communicate with the WordPress servers to check for new versions, and to both install and update WordPress core, themes or plugins.
Error Message - Your site is unable to reach WordPress.org at 198.143.164.251, and returned the error: cURL error 7: Failed to connect to api.wordpress.org port 443: Connection refused
Environment
First of all, check that you can browse to https://api.wordpress.org yourself - you should see a page with documentation, if not then the problem may be the WordPress API.
Next, check the normal sympl or root user can retrieve pages from there with wget -q -O - https://api.wordpress.org - you should get a page of text, if not there may be a problem with the network for the hosting.
Finally, if the server configuration was upgraded from Symbiosis, check that the firewall is not configured to block web requests from the www-data user - this was enabled by default in Symbiosis Jessie, so check for and remove it with find /etc/sympl/firewall/outgoing.d -name '50-reject-www-data' -print -delete ; sympl-firewall.