As an added information:
The following is the cron log when the API error (403) appears in the System Logs Dashboard
(403) Ticket denied
PHP Fatal error: Uncaught Error: Call to undefined method osTicket\Mail\Fetcher::getHost() in /var/www/html/include/class.mailfetch.php:105
So we've tried to comment out the above line and then the cron log only reports
(403) Ticket denied
and the following message appears in the System Logs Dashboard
Mail Fetcher Excessive errors processing emails for 1/2. Please manually check the inbox.
I guess the point is:
why this Mail Fetcher Error occurs only when the Reject filter is applied?
Thank you