Just wanted to let everyone know we are aware of an issue when upgrading to 2018.3. If you have the Unity Ads SDK from the Asset Store, when you upgrade to 2018.3, the Ads SDK from the Package Manager will still be included, which gives you the following error: PrecompiledAssemblyException: Multiple precompiled assemblies with the same name UnityEngine.Advertisements.Editor.dll included for the current platform. Only one assembly with the same name is allowed per platform. The solution is to remove the Ads package from Package Manager, but unfortunately, the compiler error will prevent the Package Manager UI from rendering correctly. The workaround is to remove the package manually from the /Packages/manifest.json file. I apologize for any inconvenience this has caused.