2018.3.5f1 : "DirectoryNotFoundException"

Unity Engine
1

Upgraded Unity from:
2018.2.20f1
to
2018.3.5f1
And now I get heaps of errors saying:

DirectoryNotFoundException: Could not find a part of the path “F:[…ProjectAddressHiddenForPrivacy…]\Library\PackageCache\com.unity.package-manager-ui@2.0.3\Tests\Editor\Unity.PackageManagerUI.EditorTests.asmdef”
System.IO.FileStream…ctor (System.String path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share, System.Int32 bufferSize, System.Boolean anonymous, System.IO.FileOptions options) (at :0)
System.IO.FileStream…ctor (System.String path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share, System.Int32 bufferSize, System.IO.FileOptions options, System.String msgPath, System.Boolean bFromProxy, System.Boolean useLongPath, System.Boolean checkHost) (at :0)
(wrapper remoting-invoke-with-check) System.IO.FileStream…ctor(string,System.IO.FileMode,System.IO.FileAccess,System.IO.FileShare,int,System.IO.FileOptions,string,bool,bool,bool)System.IO.StreamReader…ctor (System.String path, System.Text.Encoding encoding, System.Boolean detectEncodingFromByteOrderMarks, System.Int32 bufferSize, System.Boolean checkHost) (at :0)
System.IO.StreamReader…ctor (System.String path, System.Text.Encoding encoding, System.Boolean detectEncodingFromByteOrderMarks, System.Int32 bufferSize) (at :0)
System.IO.StreamReader…ctor (System.String path, System.Boolean detectEncodingFromByteOrderMarks) (at :0)
System.IO.StreamReader…ctor (System.String path) (at :0)
(wrapper remoting-invoke-with-check) System.IO.StreamReader…ctor(string)
System.IO.File.ReadAllText (System.String path) (at :0)
UnityEditor.Scripting.ScriptCompilation.EditorCompilation.LoadCustomScriptAssemblyFromJson (System.String path) (at C:/buildslave/unity/build/Editor/Mono/Scripting/ScriptCompilation/EditorCompilation.cs:463)
UnityEditor.Scripting.ScriptCompilation.EditorCompilation.SetAllCustomScriptAssemblyJsons (System.String[ ] paths) (at C:/buildslave/unity/build/Editor/Mono/Scripting/ScriptCompilation/EditorCompilation.cs:681)
UnityEditor.Scripting.ScriptCompilation.EditorCompilationInterface:SetAllCustomScriptAssemblyJsons(String[ ])

And I get this on almost all my projects where as I have never, ever, seen this error once before.
Updated Visual Studio to the newest version - problem persists.
Created a new project and copied the individual files in manually without including meta files and the problem still persists.

2

I have the same problem, even simply by creating an empty project and trying to run it. I uninstalled and reinstalled about 3 times, deleted folders suggested in other solutions, and nothing works.
In my case, all the errors are related to the “Packages” Folder, that I never asked to install.