My best guess would be that either the switch to Internet Explorer OR the typing of my account name by hand (INSTEAD of copying & pasting the account name) is what fixed the problem for me.
(also seems capitalization on GitHub account name does matter, but clicking Save Changes will then change it to all lower-case… this DOESN’T matter and it only matters if you get an error saying that the GitHub account name doesn’t exist when you click “Save Changes”)
I’ve had it work on two separate accounts that I was having issues with! (initially created a second set of Epic & GitHub accounts after the first accounts didn’t work)
I hope this works for everyone else having issues, best of luck!