Hi Simon and all,
In my case, it was because I replaced the "zap" address with some other value under "Addresses permitted to use the API". I noticed in the logs how the request was using "zap" which was no longer available. I renamed that address instance back to "zap", and the API started working as before - as a charm
Tested on the latest version 2.14.0
Thank you all for all the effort placed in developing and maintaining this wonderful tool!