There is another scenario where this issue reproduces (as in my case). When THE CLIENT REQUEST doesn't contain the right extension on the url, the controller can't identify the desired result format.
For example: the controller is set to respond_to :json
(as a single option, without a HTML response)- while the client call is set to /reservations
instead of /reservations.json
.
Bottom line, change the client call to /reservations.json
.