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 find the cause of the error, you can use the following steps:
1. Ensure you have a backup of your site in case there are any issues when disabling/enabling plugins or your theme
2. Disable all plugins (except your calendar plugin and Event Calendar Newsletter), and switch to a default theme like Twenty Twenty then verify the issue is resolved
3. Re-enable your plugins and switch back to your previous theme one by one verifying if the issue is still occurring, and note which one causes it to break
Once you see which one is causing the issue you can 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.