Skype, MS Teams, Slack and other applications may be blocked by SID 6855 - PROXY-ACCESS Ultrasurf.
A new signature was released to block Ultrasurf but this is impacting other applications.
Update as of 7/4/19 2:30 pm EST: The signature (SID 6855) has been removed from the Ultrasurf application. If you continue to experience issues with blocked apps/websites please update the App Control database by logging into the firewall and navigating to MANAGE | Rules | App Control and click Update.
Disable the Ultrasurf Signature Under Proxy-Access category Signature ID: 6855 should make the impacted applications working again.