This question already has an answer here:
When I go to www.askubuntu.com I get an error:
When I click advanced it says:
This server could not prove that it is www.askubuntu.com; its security certificate is from stackexchange.com. This may be caused by a misconfiguration or an attacker intercepting your connection.
This is on Google Chrome - Ubuntu 14.04, but has been reproduced on Firefox - all Ubuntu.
Just plain old askubuntu.com is fine.
Edit: Now working sometimes, but not always. Same error.