One of the advertised features of the Ithemes Security plugin is that it can be used to block access to the Rest API. It would be worth confirming that this isn’t the cause of the issue.
You don’t need to use the Sync Comment Data webhook with the WP Discourse plugin. The main benefit of using it is that it allows comments to show up immediately after they are posted on Discourse instead of having to wait up to 10 minutes to fetch new comments. On very busy sites, the webhook can reduce the number of API requests that your site is making to Discourse. This mainly affects sites that display comment numbers on a post index page.