Google’s John Mueller noted on Google+ that some webmasters may run into a hiccup when trying to use the download API for the Google Search Console CSV downloads.

John said the fix is to update the “App-Name” / “source” that you use to log in so that it matches the format specified over here.

That should then let you begin downloading the data again.

Google is coming out with a new Search Analytics API and John mentioned again that if you want into the beta, you should drop him a note on Google+ with your email address.

John said the beta should be coming out “soon.”

Forum discussion at Google+.