Usually, updating WordPress is the solution but in this case, your server is blocking or is not supporting the
(text/html`) MIME type for stylesheet. This might be due to a security configuration.
We will need for you to cop your site over to staging environment because we need to see if Pro is conflicting with your server and no third party plugins is causing this.It's not possible to investigate in your live site as there is a risk of breaking it and troubleshooting will affect your site's operation.
After you've done that, please give us the staging site's WordPress Admin and FTP access.