When I first created and rolled out various Sites from my 10.6.1 portal in late July, there was no problem logging on. Then, users started seeing a dialog box that said "Sign In" but clicking on Sign In wouldn't do anything.
My workaround is to back the URL back to https://mydomain.com/portal, have them sign in, then open the Portal Site shortcut. That worked for a while, still works for individual maps, but doesn't get them back into the Portal Site. What is the problem? It does the same thing for all browsers I've tried.
I'm using existing enterprise users security store.