Based on version 3.2.0.11 of the Squid proxy server, it is not possible to perform an Avira Update. This happens only if the proxy server is explicitly deposited within the Avira configuration or Avira determines the proxy server by default.
The reason for this is a fix for a "Bypass of browser same-origin access control in intercepted communication" vulnerability (CVE-2009-0801), which unfortunately is incompatible with the method the Avira updater uses.