If you launch the editor with the My Assets section of the Package Manager open, you'll receive the following log in the console once the asset list has been refreshed.

[PackageManager] Error User not logged in
UnityEditor.PackageManager.UI.PackageManagerWindow:OnEnable()
UnityEditorInternal.InternalEditorUtility:LoadSerializedFileAndForget(String)
UnityEditor.WindowLayout:LoadDefaultWindowPreferences()

The Package Manager will also display an error saying "Error fetching packages, see console". Manually refreshing the package list at this point will NOT clear the error, even though the package list refreshes correctly.

I cannot reproduce this on 2020.1, so looks 2019.3 specific.

1. Open Unity and create a new project
2. Ensure that the Package Manager window is open and is set to the My Assets Tab
3. Close the editor
4. Open the editor again and open the previous project
5. When the editor opens, wait for the Package Manager to refresh

Expected Result:
The Package Manager completes the refresh process with no errors.

Actual Result:
The Package Manager displays a User Not Logged In error even though the packages are refreshed correctly.

Tested on Windows 10.

Reproduced on 2019.3.0f5, 2019.3.0b2

Cannot reproduce on 2020.1.0a19, 2019.3.0a12

Blocked from testing on 2019.3.0b1