This is similar/same as a previous thread from 2015, but something I just noticed on our firewalls. The logs are full of 407 errors, almost one for every 200 code when I crunch the numbers.
Standard Proxy with AD SSO
DHCP autoproxy configuration option 252 = http://sophos.internal.com:8080/wpad.dat
Group created in Definitions and Users for domain users.
Authentication configured and tested.
We get the same result with all browsers (IE, Edge, Chrome, Firefox) whether they are configured with "Automatic Discovery" (DHCP - Except Firefox which doesn't support DHCP discovery), "Automatic Proxy Configuration" (http://sophos.internal.com:8080/wpad.dat) or "Manual Proxy" (http://sophos.internal.com:8080).
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