Remote Desktop Troubleshooting: Renew the RDP Self-Signed Certificate
There is an RDP Self-Signed certificate after enabling the RDP on Windows servers & "Remote Desktop Configuration service" is responsible for that.
Certificate Store: Cert:\LocalMachine\Remote Desktop\
If that Certificate is expired, you need to delete the certificate and restart the Remote Desktop Configuration service. That will create a new certificate.
net stop SessionEnv
net start SessionEnv