Opened a ticket with support.
They have not heard of this specifically, but I was told that it may require up to three restarts before everything is back to normal following the update to Endpoint client 21.5.0.19.
In this case, I did just that - and of course found two very interesting things.
1) I downloaded Symhelp (just in case) and received the message: "SymHelp.exe has no available reputation information". I just cracked up at that one.
2) Saw this message when I tried to update the client directly:
This, of course, is a bit more disturbing. Obviously there is no "OK" button, but I am wondering what functionality is not working properly behind the scenes. Eventually, it starts to process; and on the third reboot, it actually pulled down new definitions.
Gonna start to look at Sophos as an alternative, because this is starting to get to me...