Why the "Server is down" during avast update?

If you're trying to update your avast! antivirus programme, but the update can't complete because of the "Server is down" error one the following problems may have occured:

• Your Internet connection is interrupted or broken. Check your Internet connection by visiting any website. If it doesn't open contact your Internet service provider.
• Your firewall settings block connections to avast! update server. Review your firewall settings and check connectivity.
• If you're using a proxy server, make sure you configured avast! correctly to use that proxy. Go to Settings » Update (BASIC) » Details » Proxy » Specify Proxy Server