KB4503267 is meant to fix problems with TFTP downloads with Variable Window = true, but over here it iscausing those problems, for downloads with Variable Window = false.
This is on Server 2016.
We are using in-house designed ARM CPU devices booting from the network. They are built up around a Digi ConnectCore 7U module running an image built on Digi's own Net+OS. The boot loader is a minimal Net+OS build that just downloads a larger image to RAM and starts it, by rebooting to that RAM image.
We have been using these devices with different TFTP servers for more than 10 years, and they have always worked fine.
Now they suddenly start booting flaky (guesstimate: about 1 boot attempt in 3 to 5 fails) and I have dozens of these in event log:
Applications and Services Logs \ Microsoft \ Windows \ Deployment-Services-Diagnostics \ Admin
Event ID = 4101
The Following Client failed TFTP Download: Client IP: 10.11.19.150 Filename: image.bin ErrorCode: 644 File Size: 1002330 Client Port: 18000 Server Port: 59082 Variable Window: false
Before the update was installed I had on average less than one of these events per month, now I have dozens per day.