With two unrelated apps - that had been working for years, these stopped working last week (noticed on different days but around and before 30 May) due to a change in the behaviour of URLs to Backend Workflows.
Where previously a URL like this to a Backend API would have worked (notice the 2 slashes)
https://myapp.io//api/1.1/wf/someapi
This will now return a 404 (not found) and needs to be corrected to
https://myapp.io/api/1.1/wf/someapi
Hopefully this tip will save someone a lot of pain.
The error will only be observed outside of Bubble so no point looking in your Bubble logs wondering why the API call is no longer appearing in your Bubble logs