On my Ubuntu 16.04 LTS, when I try to browse askubuntu.com with Opera VPN, I receive Page Not Found error message as shown below, I wonder why this is happening.
When Opera VPN is enable, I receive Page Not Found error for any question on askubuntu.com, for example this question or any other question.
When Opera VPN is disabled, there is no error message.
This problem is common to accessing all Stack Exchange sites with Opera VPN, but it does not affect the https version of the URL, it only affects the http version. It should be possible to access the page you linked to in your question by replacing http with https in the URL.
From the accepted answer to Problems with accessing SE sites with Opera VPN at Meta Stack Exchange:
Our sites work well on most VPN solutions.
We can't fix a broken VPN solution.
What you are describing is the VPN completely blocking access to our site - there is nothing we can do to change it.
I suggest reporting this to Opera so they can fix the issue.
httpwithhttpsin the URL worked – Megidd Apr 13 '17 at 09:20