@simon-muller,
The wpDiscuz Frontend Moderation sends the JSON data, but when data is returned it can not parse from JSON, that caused the issue.
The probable reason may be a wrong server configuration or the plugin conflict.
You can create a create a staging copy of your website and try to find the reason of the issue, by deactivating the plugins and check the server configuration.
If you will not be able to find the reason of the issue, just contact us via support[at]gvectors.com email address and ask for a refund.
Also please set the email subject " refund.