aaPanel_Kern
The problem was solved by restarting the server.
However, I have opened the thread because I am interested in the cause of the problem. Fail2ban stopped working without warning. I would like to know why, how to prevent this failure and how to fix it in the future without restarting the server.
I would also like to know why the paused status of the application was not reflected from the home page of the panel, in case something went wrong with my configuration.
Any help would be welcome. Thanks!