Skype for Business Screen Sharing

We're running a 90D on 5.2.7 with URL Filtering.  Not using any traffic shaping or app control.

Some of our customers use Skype for Business, and ask us to sit in on presentations.  We get a connection just fine, but whenever they try to share their screen we get an error popup saying "network problems have interrupted the presentation".  I tried putting all the MS sites listed in their KB into our whitelist, but for some reason after that, our users stopped being challenged to log in (all I can figure is one of the MS listed sites - maybe verisign? - overrides our login policy and lets them pass through.

Any ideas?

Thanks