RSS error message: "No feed items exist on this RSS feed"

Hi friends,

We have created an RSS feed at this address, using RSSUnify.com:

- https://feed.rssunify.com/5c53a4afa9249/rss.xml

The feed is working and is accessible from feed readers, however when we try to use the RSS Widget and we paste this address we receive an error message: "No feed items exist on this RSS feed".

Why can that be? Do we need to have a specific configuration elsewhere?

Thanks.