RSS feed won't display

Hello world,
Ik have a weird issue that i will try to describe as clear as possible. I wanted to display the latest posts from the Mingle-Forum plugin in a widget on the right side of my blog.

This is the feed URL: https://www.23dingen.nl/ndcvbk/wp-content/plugins/mingle-forum/feed.php

Since this URL is in a secure website, i made an alternative URL via Freemyfeed.com, which results in this ‘freed’ (shortened)URL: https://goo.gl/wgcnz
You will see, that this URL works fine, except from the fact that you need to login to view the actual article.
I am also able to subscribe to this feed in Google Reader.

Now, when i use this URL in a RSS widget – i only get errors like:

RSS Fout: WP HTTP Error: error setting certificate verify locations

Thanks for any help.
Rob