Project in a complete mess after upgrade to 2020.1.0f1

I (unfortunately) upgraded to 2020.1.0f1 and the Package Manager is now totally f***ed up and Unity is just in a mess. When I build my project I get 1000+ build errors like:

Error CS0234 The type or namespace name 'UI' does not exist in the namespace 'UnityEngine'
Error CS1069 The type name 'Rigidbody' could not be found in the namespace 'UnityEngine'.

The Package Manager is very odd.

• The "In project" list contains none of the custom packages I use (but a number of standard packages like UI).

• The "My Assets" list seems to be ok, but no matter what I do there, the packages never show up in "In project". I take a package that is definitely already in the project and import it. I get a message saying that the import was done ok. If I try to import it again I get a message saying that it is already in the project. But it still doesn't show up in the "In project" list so I have no chance to uninstall it (or doing something else).
• The "Built-in" list has everything (packages like UI and others) enabled even though the Editor.log recommend that I should enable packages there.

The editor log says:

The following packages were not registered (reason: failed entitlement check).
com.unity.collab-proxy@1.3.8
com.unity.ide.rider@1.2.1
com.unity.ide.visualstudio@2.0.2
com.unity.ide.vscode@1.2.1
com.unity.test-framework@1.1.14
com.unity.textmeshpro@3.0.0-preview.1
com.unity.timeline@1.3.4
com.unity.ugui@1.0.0
com.unity.modules.ai@1.0.0
com.unity.modules.androidjni@1.0.0
com.unity.modules.animation@1.0.0
com.unity.modules.assetbundle@1.0.0
com.unity.modules.audio@1.0.0
com.unity.modules.cloth@1.0.0
com.unity.modules.director@1.0.0
com.unity.modules.imageconversion@1.0.0
com.unity.modules.imgui@1.0.0
com.unity.modules.jsonserialize@1.0.0
com.unity.modules.particlesystem@1.0.0
com.unity.modules.physics@1.0.0
com.unity.modules.physics2d@1.0.0
com.unity.modules.screencapture@1.0.0
com.unity.modules.terrain@1.0.0
com.unity.modules.terrainphysics@1.0.0
com.unity.modules.tilemap@1.0.0
com.unity.modules.ui@1.0.0
com.unity.modules.uielements@1.0.0
com.unity.modules.umbra@1.0.0
com.unity.modules.unityanalytics@1.0.0
com.unity.modules.unitywebrequest@1.0.0
com.unity.modules.unitywebrequestassetbundle@1.0.0
com.unity.modules.unitywebrequestaudio@1.0.0
com.unity.modules.unitywebrequesttexture@1.0.0
com.unity.modules.unitywebrequestwww@1.0.0
com.unity.modules.vehicles@1.0.0
com.unity.modules.video@1.0.0
com.unity.modules.vr@1.0.0
com.unity.modules.wind@1.0.0
com.unity.modules.xr@1.0.0
com.unity.modules.subsystems@1.0.0
com.unity.modules.uielementsnative@1.0.0
com.unity.ext.nunit@1.0.0

Not having English as native language I don't fully understand what "entitlement check" means, why it fails and if this has anything to do with the problems I see.

The project is completely unusable right now and I don't have a clue where to start look for the problem. I have of course tried to restart Unity and the computer a number of times. I have run the "Reimport All" function in Unity without any change. I have built and rebuilt my project without any change. None of the projects in my solution reports any broken references.

Please, Anyone. Help.

 

https://forum.unity.com/threads/visibility-changes-for-preview-packages-in-2020-1.910880/

Look here for some info about the package manager changes

 

Yeah I would recommend not upgrading to 2020 right now, as you can see it's a mess. Hopefully you're using version control and can easily revert, or you made a backup?

 

No version control and no backup (of the latest changes)