Looking back through the build log, I do see some Xbox related errors:
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(28): error C2039: 'Xbox': is not a member of 'Microsoft'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: C:\Program Files (x86)\Windows Kits\10\Include\10.0.15063.0\winrt\wrl\event.h(1195): note: see declaration of 'Microsoft'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(28): error C2039: 'Services': is not a member of 'Microsoft'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: C:\Program Files (x86)\Windows Kits\10\Include\10.0.15063.0\winrt\wrl\event.h(1195): note: see declaration of 'Microsoft'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(28): error C2039: 'System': is not a member of 'Microsoft'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: C:\Program Files (x86)\Windows Kits\10\Include\10.0.15063.0\winrt\wrl\event.h(1195): note: see declaration of 'Microsoft'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(28): error C3083: 'Xbox': the symbol to the left of a '::' must be a type
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(28): error C3083: 'Services': the symbol to the left of a '::' must be a type
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(28): error C3083: 'System': the symbol to the left of a '::' must be a type
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(28): error C2039: 'GetTokenAndSignatureResult': is not a member of 'Microsoft'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: C:\Program Files (x86)\Windows Kits\10\Include\10.0.15063.0\winrt\wrl\event.h(1195): note: see declaration of 'Microsoft'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(28): error C2061: syntax error: identifier 'GetTokenAndSignatureResult'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(172): error C2039: 'Xbox': is not a member of 'Microsoft'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: C:\Program Files (x86)\Windows Kits\10\Include\10.0.15063.0\winrt\wrl\event.h(1195): note: see declaration of 'Microsoft'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(172): error C2039: 'Services': is not a member of 'Microsoft'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: C:\Program Files (x86)\Windows Kits\10\Include\10.0.15063.0\winrt\wrl\event.h(1195): note: see declaration of 'Microsoft'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(172): error C3083: 'Xbox': the symbol to the left of a '::' must be a type
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(172): error C3083: 'Services': the symbol to the left of a '::' must be a type
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(172): error C2039: 'XboxLiveContext': is not a member of 'Microsoft'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: C:\Program Files (x86)\Windows Kits\10\Include\10.0.15063.0\winrt\wrl\event.h(1195): note: see declaration of 'Microsoft'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(172): error C2143: syntax error: missing ';' before '^'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(172): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(172): error C2238: unexpected token(s) preceding ';'
UATHelper: Packaging (UWP (x64-64bit)): UnrealBuildTool: E:\UEUWP\MICROSOFT_UWP_UNREAL\Engine\Plugins\Online\XboxOne\OnlineSubsystemLive\Source\Public\OnlineSubsystemLive.h(173): error C2039: 'Xbox': is not a member of 'Microsoft'
There were more of them.