Dynamics 365 (CRM) on-premise cannot login using Plugin Registration Tool or Custom App using Tooling connector.

If all of a sudden you are unable to login via PluginRegistrationTool or other apps, reason can be the recent windows security update on CRM Server.

2020-01 Cumulative Update for Windows Server 2016 for x64-based Systems (KB4534271)

Removing this update restored our connection 🙂