It seems to be a certificate issue on the Exchange server. Try to delete the old certificate and then restart IIS. For more details, please read Kismet’s reply in this link.
Besides, Jeff Schertz’s blog also gives detailed steps for integration between Exchange and Skype for business. You can refer to this link: https://blog.schertz.name/2015/09/exchange-and-skype-for-business-integration/.
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