I’ve had these issues as well. Specifically, not being able to update a SmartApp’s settings, editing an existing device, and adding a new device. And they’re intermittent - every once in a while it works, but usually I get the error. Per @Ryan780’s comments, I tried using a Chrome Incognito tab, and that worked for me, too!
I did contact SmartThings Support, but so far they’ve only suggested logging out and back in, which I’ve tried a zillion times but doesn’t help. Seems like so far they don’t know what the root cause is.