Why would Windows OS RDP thumbprint change?

First, I am a Linux admin, so bare with me if I am using the wrong terminology.

We have a Windows 2008R2 server that I routinely have to remote into for some tasks.  I use freerdp from a Linux system to remote to the system.

Recently I received an error when trying to remote to the system that said the fingerprint has changed.  I had an admin log in and confirm that the thumbprint has indeed changed.  

My question is, what would make the thumbprint change?  We have had no reinstalls, no cert updates, etc...
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Svet PaperovIT ManagerCommented:
RDP uses SSL to secure its connection. A self-signed RDP SSL certificate expires each 6 months. Windows remote users receive a warning each time a new self-signed RDP SSL certificate is generated and they can chose to accept it or to disconnect.

If you know a little bit about SSL certificates, you know that the fingerprint corresponds to the public key of such certificate.
savoneAuthor Commented:
Is there a way to extend the expiration date beyond 6 months?  And can we use a cert from a CA (verisign?) for RDP SSL connections?
Svet PaperovIT ManagerCommented:
Yes, if you purchase a SSL certificate from a public CA you won’t get any warnings about the fingerprint because the certificate chain will be trusted by your computer.

I found Verisign a little bit more expensive than DigiCert (my preferred), Comodo or GoDaddy. But it depends on your requirements.

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