Looks like there are some new IP addresses that need to be accounted for Skype for Business to work w/ SSL Decryption enabled.
Here is the MS link plus some of their verbiage, https://techcommunity.microsoft.com/t5/Skype-for-Business-Blog/Updated-IP-ranges-and-ports-for-Skype...
A similar topic came up previously at the below link.
I added the two ranges to not be decrypted and users were able to successfully authenticate to Skype thereafter. I saw in the normal monitored traffic tab that traffic was being permitted but nothing was super obvious that it was a SSL Decrypt issue. Hope this helps others out.
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