Hi,
We got the accesses and did some debugging. From that, we concluded that there is something blocking outgoing connections on your server. It's probably some security system of your hosting.
It not only blocks the connection to the European Central Bank feed, but even if we change the feed to the Yahoo finance feed (there is a setting in the plugin for that), it also returns no data back.
So the fact that you have the issue for two different feed, and that these feed work fine for me (if I test on my local website for example, both feed refresh the rates properly) and other users, indicates that something is blocking outgoing connections.
I'm afraid you'll have to check with your hosting support so that they can investigate and tell you what can be done.
You can tell them that the code:
<?php echo file('https://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml');
should display the XML feed from the European Central Bank but on your hosting when you do that it doesn't work.
For example, I've created a file test.php with that code at the root of your website. So you can go to
yourwebsite.com/test.php
and you should see the XML feed.