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IIS is Internet Information Services, the web server included with Windows Server operating systems. All current versions are built on a modular architecture; modules can be added or removed individually so that those required for specific functionality are installed. The full installation of IIS includes HTTP, security, content, compression, caching, logging and diagnostics.
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I would suggest to check the certificate and see it is valid www.sslshopper.com/ssl-checker.html (just give the external access of you citrix access). I am sure that this will give some clue where the issue actually lying.