We are still working out some issues that the WordPress 5.5 update created. Many themes and plugins experience problems with this update.
For the time being, until we release a new version we recommend to installing the “Enable jQuery Migrate Helper” free plugin https://wordpress.org/plugins/enable-jquery-migrate-helper/ which should fix these problems temporarily.
We tested both plugins and didn’t find any issues with it.
There’s a new version available of Seasonable pro which should be fully compatible with WP 5.6
Perhaps instead of SiteOrigin CSS you can use the native WP Customizer / Additional CSS option
There’s no need for additional plugins to add custom CSS in WordPress as it’s core functionality for a long time.