We are seeing this error when accessing the "API Features" page for one of our titles: "There was an error while loading data from the server. Please reload the page to retry."
For another title (same account), we do not see this error, and instead see the usual content of that page.
So it looks to be something related to the title's data, but without more information, it's not something we can debug on our own (especially if fixing it requires changing data that is only editable via the API Features page).