TF215097 when running a build

I have been working on one of our build boxes today restructuring our Surface solutions to make better of Nuget. This involved upgraded the Azure SDK on the build box to the new June release, which needed a reboot halfway through the process. After the reboot and tried a new build I got the error

TF215097: An error occurred while initializing a build for build definition \Surface\RetailApplication.Main CI: Could not load file or assembly ‘System.Drawing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a’ or one of its dependencies. The located assembly’s manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

Basically the build did not start at all. Various forum posts point to corrupt build template .XAML or missing assemblies. But as it was working, the assembly name is in the core framework it all seem a bit strange.

The fix was the old favourite, stop and restart the build service from with TFS Admin console on the build box. Once this was done all as fine, so I guess some rubbish was cached.