By default, FastCron will check your cronjob URL's returned HTTP status code. See the List of HTTP status codes.
Only those URL returns HTTP status code 2xx Success (e.g. the most popular is 200 OK) is considered as a success. If your cronjob URL returns another HTTP status code, your cronjob will be marked as failed.
In case you believe that your cronjob works fine even when it returns that HTTP status code, you can just make FastCron silently ignore HTTP status code checking. Just click Edit cronjob, select the section Detect failure and notify you, then check the box Test cronjob.