Installed Unity 2019.3.0f1 (RC) today, installing some plugins from the asset store to check compatibility. After installing a plugin (Quantum Console) I then get tons of errors from the Packagemanager. First thing I did deleting that plugin. Didn't help. Restarted Unity. Didn't help. Closed Unity and deleted the Library folder. Didn't help. Had a closer look at the errors, one of them like this: Library\PackageCache\firstname.lastname@example.org\Editor\Sources\UI\Common\UIUtils.cs(1,32): error CS0234: The type or namespace name 'UIElements' does not exist in the namespace 'UnityEngine.Experimental' (are you missing an assembly reference?) Thought "Hm, UIElements?" - installed the UI Builder preview package. Didn't help. Deleted ALL previous imported plugins. Closed Unity. Deleted Librarby Folder again. Unity reimports. Didn't help, tons of errors still showing. Looked closer into the errors again. Since it came from the PackageManager UI package I removed this package. Helped. All errors gone. But...how can install this package again. Afaik this package is needed to display the actual Package Manager but...I still can access the Package manager window, remove/install packages. So I'm a bit confused. Why did the Package Manager throw all these errors all of a sudden? How can I install the Package Manager UI Package after removing it, it doesn't show up as a package? Why am I still able to access the Packager Manager without the respective UI Package?