Active Roles Skype for Business policies are enabled when it isn't being used, and is impacting the Approval Notifications from sending.
The Active Roles Verbose logs will show the following:
[SkypeforBusiness][SkypeforBusinessConnection::_TryEstablishConnection] 'Connecting to remote server xxxx.xxxxx.xxx failed with the following error message : WinRM cannot complete the operation. Verify that the specified computer name is valid, that the computer is accessible over the network, and that a firewall exception for the WinRM service is enabled and allows access from this computer. By default, the WinRM firewall exception for public profiles limits access to remote computers within the same local subnet. For more information, see the about_Remote_Troubleshooting Help topic.'
This issue may also cause a slow response time when selecting the Modify... button to customize the notification message.