Hi gcalabro, Sorry about the issue. It’s a bug and we will update it soon.
But now you can remove a line of code from partials/section-slider.php ( line no: 70 )

$header_text_class = sanitize_html_class($header_text_class);

just remove this line and it will work properly. If you get stuck the please provide your site url and login credential and we will fix it for you.