I'm having the exact same issues on the same timeline in our organization. We have a large number of clients that appear to be patching just fine, but a subset that simply started to fail the scanagent with error 0x8024400d during the first week of December. We've also tried reinstalling the WUAgent, repairing WMI and uninstalling/reinstalling the SCCM client, but nothing is working for those clients. Any luck figuring out your issue?