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Hello @naratt

My sincere apologies that we could not respond earlier. We now have a dedicated support staff for supporting www.remarpro.com forum posts. Hopefully, your future support requests are answered in due time.

The issue that you’ve raised is not due to the plugin as the plugin does not play any role in the editor. It simply uses WordPress’s core post type to register a custom post type.

There could be lot of reasons why you could be facing this issue. Here is a guide to fixing this issue: https://www.wpbeginner.com/wp-tutorials/how-to-fix-the-invalid-json-error-in-wordpress-beginners-guide/

Hope this helps. Let us know if you have any other questions or need further help.

Thanks,
Ibrahim