I’ve replicated this issue using WP 5.5 so I downgraded WordPress in your staging site to 5.3 using the WP Downgrade | Specific Core Version plugin and that solved the issue. I created 4 grids to double check.
- 1st grid was created after downgrading WordPress to 5.3 while theme was switched to 2020 and all plugins deactivated except The Grid.
- 2nd grid was created after activating all plugins
- 3rd grid was created after activating X parent
- 4th grid was created after activating your child theme
Please try all of that in your live site and just note that we do not provide support the WP Downgrade | Specific Core Version nor any 3rd party plugins so you should contact the plugin author if you encounter issues or need support.
In the changelog of The Grid, version 2.7.6 includes support for WordPress 5.5 but we’ve receive similar reports stating issues encounter with the said The Grid version. There could be some variable that might have been missed. It could be a conflict with a PHP version so you could also try downgrading to PHP 7.2 and update WP to 5.5 though in my test site, even downgrading to PHP 7.2 still won’t work. I’d recommend though that you stay tuned for the next The Grid update maybe a fix will be included.
Hope that helps.