Hello,
You can reinstall the self-signed certificate on your client, making sure it's issued by the same server. If not, consider foregoing SSL authentication by disabling NLA:
a. Open the gpedit.msc applet.
b. Go to Computer Configuration -> Administrative Templates -> Windows Components -> Remote Desktop Services -> Remote Desktop Session Host -> Security.
c. Activate the "Require use of specific security layer for remote (RDP) connections" and choose RDP as the Security Layer.
d. Turn off the "Require user authentication for remote connections by using Network Level Authentication" policy.
e. Restart the Terminal server.
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