I was getting this error and I had to clear all of my browser cache to resolve it.
You can also use an incognito/private tab to login to IDE @ https://account.smartthings.com
I have to use an incognito window whenever I want to make changes in the IDE. However, I have to use a regular window for live Logging. Go figure.
Using an incognito window worked.
I get that error only when I have another browser tab open with webcore.