Did you change any other settings in your website? (eg. debug code, cron settings)?

Please do following
1. add error_log( 'pop3 event init' ); just after function pop3_event() {
2. Go to Front End PM PRO > Settings > Emails > Email Piping/POP3 then set Enable as “None” then save changes and again set as POP3 and again save changes. Then wait at least 15 minutes and refresh your website couple of times then see error log.

3. If still no log shows please install any cron plugin (eg. https://wordpress.org/plugins/wp-crontrol/ ) then go to Tools > Cron Events and see if fep_pop3_event showing under hook name. If yes give me that row screenshot. Also click “Run Now” and see again error log if any log there.

Let me know.