With the 1803 update I got a problem.
I’ve used Forefront TMG2010 and it’s predecessors for over a decade. It generally sits there doing it’s job and provides me with the means to control access in and out of the network.
The client has sat on the taskbar since XP days, without a problem. Until the 1803 upgrade.
Microsoft don’t support TMG any more, so no joy looking on the forums for support, but I did find this: https://support.microsoft.com/en-us/help/936773/yellow-exclamation-mark-appears-next-to-the-firewall-client-icon-on-a. Although I didn’t change the client (mine was v7.xxxx and 4 years younger), but the registry change had cleared the yellow marker on the TMG Taskbar icon.
I hadn’t found any actual problem with TMG (yet) that this might have caused, but at least an error has been cleared.