You are unable to connect to Dynamics 365 (online), version 9.0, using SDK tools such as the Plugin Registration Tool or Configuration Migration Tool. After providing the connection information and clicking the Login button, you are returned back to the login dialog.
This can occur if you are not using the latest version of the SDK tools which use TLS 1.2. Dynamics 365 (online), version 9.0, requires TLS 1.2 or later. TLS 1.0 and 1.1 are no longer supported.
For more information on the change to requiring TLS 1.2, see the following article: