No syntax error in SwaggerHub - but cannot download a resolved YAML file

Hi,

I have a valid Swagger definition (or at least SwaggerHub doesn't report any errors) - but when I try to download the resolved YAML file (or the resolved JSON file) an error is displayed:

How do I find out where the error is?

(I am aware that I have a definition that is declared but never used - but that can hardly be the problem!?!)

Link to API: https://app.swaggerhub.com/apis/dcsaorg/DCSA_TNT/2.1.0