Global Posts Feed note being recognised.

Hi there

I have just installed Global Posts Feed into my multisite network but when I try to activate the RSS on a widget, i get the following message:

RSS Error: A feed could not be found at https://my site name/feed/globalpostsfeed A feed with an invalid mime type may fall victim to this error, or SimplePie was unable to auto-discover it.. Use force_feed() if you are certain this URL is a real feed.

(NOTE I have deliberately replaced my actual site with "my site name" as I don't want the site name to be public.)

I suspect I am doing something wrong - any ideas?

Thanks