Hello Jerry,
Yes, I think that there is some kind of local “snag” that is preventing you from accessing that specific webpage.
As a last resort, you could try to re-burn the Shake microSD card (instructions here: Rest Raspberry Shake - #2 by Stormchaser to see if that was the cause of the problem. I don’t think it is, but since it seems we are stuck, it could be worth a try.
You can also try to connect (momentarily) the Shake to another local network (a friend, neighbour, colleague) that has a different modem/router, and see if, from there, you can access rs.local/ and then configure the Shake, before bringing it back to your home.
It’s definitely a puzzlement, I cannot remember a similar issue in the past, but as the saying goes, there is always a first time.