I suspect the error you are getting could be because of network issue. Could you please check if there is no network related issue.
If you still face the issue, please provide some more details like what steps you are following .
But i can see the editor working in other accounts, I am just creating a landing page and using the page activity to design. Whenever i open the page activity in the web app this error pop up only in a particular instance.
What do you mean by other accounts?
I'd suggest to try one of the following:
Let us know if you find the root cause of this,