Package fails due to Unknown Cook Failure

When trying to package my project for Windows 10 64-bit, I get

UATHelper: Packaging (Windows (64-bit)):   LogInit: Display: LogOutputDevice: Error: begin: stack for UAT
UATHelper: Packaging (Windows (64-bit)):   LogInit: Display: LogOutputDevice: Error: === Handled ensure: ===
UATHelper: Packaging (Windows (64-bit)):   LogInit: Display: LogOutputDevice: Error:
UATHelper: Packaging (Windows (64-bit)):   LogInit: Display: LogOutputDevice: Error: Ensure condition failed: TickMode == ETickMode::Automatic [File:D:/Build/++UE4/Sync/Engine/Source/Runtime/UMG/Private/Components/WidgetComponent.cpp] [Line: 942]

I see this several times in the output, and I don’t have an idea as to where it might be coming from.