Have a customer with CRM 2015, IFD configured. We configured this about 2 weeks ago without issue, and is still working. However, when we try to connect a console app to CRM via IFD (e.g. Plugin Registration Tool, XRM Toolbox, even ADFS 2.0->Update Federation Metadata) we get "The underlying connection was closed: An unexpected error occurred on a receive," "Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host," and "Metadata contains a reference that cannot be resolved 'https://url.company.com/XRMServices/2011/Discovery.svc?wsdl.'" We can browse to the discovery service in IE without issue. A fiddler trace taken during this transaction shows "HTTPS handshake to url.company.com failed. System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host."
I have tried reboots and repairing CRM; there are no corresponding errors in a server trace or the event logs on the server.