When you try to add your events into your newsletters automatically, you may encounter Mailchimp or other programs saying your "RSS feed" is invalid. Event Calendar Newsletter uses RSS feeds in order to transmit the details of your events and any errors will cause this to fail.
A plugin or theme is outputting extra whitespace/characters
Your theme or a plugin installed on your site may be incorrectly outputting extra characters when it shouldn't. Any extra output will cause the RSS validation to break.
To fix this, disable all plugins except for Event Calendar Newsletter and your WordPress calendar plugin, and switch to a default theme like Twenty Sixteen. Try the feed URL again. If it works, re-enable plugins one by one along with your theme and test each time you re-enable one to see which one is causing the issue then contact the author of that theme or plugin to have them fix it.
Feed output as text/html
If you run your feed URL through the RSS validator and get a message like:
This feed is valid, but interoperability with the widest range of feed readers could be improved by implementing the following recommendations.
Your host is telling Mailchimp and others that the feed is not of content type "rss+xml" but rather text/html through some kind of cache.
You'll need to contact your hosting company to see how this can be altered so Mailchimp and others can detect it as a valid RSS feed. Alternatively you can try and use something like FeedBurner to consume the Event Calendar Newsletter feed and output something Mailchimp and others will recognize.
With Event Calendar Newsletter you'll be able to send newsletters with your upcoming events quickly and accurately.