Upload of fallback file fails - Bad JSON response

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Just installed Centova Cast 3 on Ubuntu 16.04 and tried to upload a fallback file and I'm getting the following error:

4.1MB, Error: Bad JSON response: SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data

This is with an mp3 and a wave file.

Any ideas what the problem could be?