Symptom:

1) You are using Expression Web to publish a site, and receiving the following error:

The server couldn't log you on because you are connecting through a proxy server, but the server's authentication scheme does not work across a proxy.

2) Checking the proxy settings, you find in the manual configuration it shows: http://localhost:9100/proxy.pac

3) you have Google Web Accelerator installed.

4) You have no other proxy involved that you know of.

If all of these symptoms are true, then your error is being caused by GWA.

Resolution:

Uninstall or disable Google Web Accelerator (GWA).    If you simply try to remove the entry in the manual proxy settings, you may cause the application or system to hang.  Instead, you should uninstall, or temporarily disable GWA... the "googlewebaccelerator.exe" process needs to NOT be running on your system when you attempt to publish.