Have you tried re-typing out the shortcode manually, to make sure the quotes aren’t “special quotes” which can happen if things are copy/pasted from somewhere?
Otherwise here may be another plugin/theme interfering or some kind of caching on your site/server. You can check for a conflict with the following:
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 this plugin), and switch to a default theme like Twenty Sixteen 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
Let me know which plugin/theme causes the issue and I may be able to come up with a fix, but you’ll also need to contact the theme or plugin author and have them fix the issue.
If this doesn’t do the trick, check you don’t have any caching on your site and try on another site to see if there’s an issue with the setup.