Hi!
We ship The Events Calendar complete with minified CSS and JS. Unfortunately, some minification tools don’t play too nicely with code that has already been minified, resulting in breakages.
The best advice here is, if possible, to set up exceptions so that any files belonging to The Events Calendar (or other related plugins) are not minified a second time.
If you’re sure the problem is only with CSS then it may be that you only need to set up exceptions for those assets and not the JS.
I hope that helps 🙂