Where do I report issues with official archive mirrors?

Yes, you should report it. Just send a mail to [email protected] with the time, ip, and log of the problem that you were facing. Or drop by the irc channel at irc://irc.freenode.net/ubuntu-mirrors

You can also report a ticket visiting https://rt.ubuntu.com or sending a mail to [email protected].

_{Thanks to Benjamin Drung (OP) and Jorge Castro for pointing this out.}

From the w3c page:

406 Not Acceptable

The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.

Unless it was a HEAD request, the response SHOULD include an entity containing a list of available entity characteristics and location(s) from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the Content-Type header field. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice MAY be performed automatically. However, this specification does not define any standard for such automatic selection.

 Note: HTTP/1.1 servers are allowed to return responses which are
  not acceptable according to the accept headers sent in the
  request. In some cases, this may even be preferable to sending a
  406 response. User agents are encouraged to inspect the headers of
  an incoming response to determine if it is acceptable.

If the response could be unacceptable, a user agent SHOULD temporarily stop receipt of more data and query the user for a decision on further actions.