I reckon this happens sometimes while adding a link using the "insert hyperlink" tool (either by clicking the button or pressing Ctrl+L) instead of using Markdown.
The input box contains https:// (which is selected) by default
(and I believe it used to have http:// by default previously). So usually while pasting the link, it overwrites the https:// with ...
If you need HTTPS to protect your privacy on all pages of Ask Ubuntu, install the HTTPS Everywhere browser extension and it will modify all requests by replacing http: with https:.
This is a weakness of Stack Exchange and should be corrected. They might need to invest money on better hardware and in software engineering to support the ...
The problem seems to be caused by copying a link with a preceding space into the Insert Hyperlink dialog. The software adds http:// in front of it and a broken link starting with http://%20https:// is born.
See What causes “http://%20https://” links to be made? on Meta.SE for details.