One thought… This might not have anything to do with it, but different smart things accounts are on different shards, which means you have to use different URLs to make things work.
Right now at the time of this writing there are three different shards.
If your account with SmartThings with set up prior to November 2015, you are probably on NA01, which would use the URLs given in the first post of this thread.
If your account with smartthings was set up after that and you have a US frequency hub, you are probably on NA02
And if you are in the UK, you are probably on Eu01
More about this in the following thread, but I just wondered if this might have anything to do with what you’re seeing. Just a thought.