votdev could you please have a look to validate the suspected root cause " "monit" service checks proftpd`s state every 30 sec and cause this error"?
Deep inspection configs and logs!
1 - Simulating problem re-enabling port 21 check in monit config...
Time in logs is corresponding
every 30 sec monit detects error and restarts proftpd
/var/log/syslog:
Jan 9 20:01:09 slavaya-omv monit[27662]: Monit daemon with pid [27662] stopped
Jan 9 20:01:09 slavaya-omv monit[27662]: '\slavaya-omv.local' Monit 5.26.0 stopped
Jan 9 20:01:09 slavaya-omv monit[10898]: Stopping daemon monitor: monit.
Jan 9 20:01:09 slavaya-omv systemd[1]: monit.service: Succeeded.
Jan 9 20:01:09 slavaya-omv systemd[1]: Stopped LSB: service and resource monitoring daemon.
Jan 9 20:01:29 slavaya-omv systemd[1]: Starting LSB: service and resource monitoring daemon...
Jan 9 20:01:29 slavaya-omv monit[10990]: Starting Monit 5.26.0 daemon with http interface at /run/monit.sock
Jan 9 20:01:29 slavaya-omv monit[10987]: Starting daemon monitor: monit.
Jan 9 20:01:29 slavaya-omv systemd[1]: Started LSB: service and resource monitoring daemon.
Jan 9 20:01:29 slavaya-omv monit[10992]: '\slavaya-omv.local' Monit 5.26.0 started
Jan 9 20:01:34 slavaya-omv monit[10992]: 'proftpd' failed protocol test [FTP] at [localhost]:21 [TCP/IP] -- FTP: error receiving data -- Resource temporarily unavailable
Jan 9 20:01:34 slavaya-omv proftpd[10994]: 192.168.254.112 (127.0.0.1[127.0.0.1]) - mod_tls.c: error initializing session: Отказано в доступе
Jan 9 20:02:09 slavaya-omv monit[10992]: 'proftpd' failed protocol test [FTP] at [localhost]:21 [TCP/IP] -- FTP: error receiving data -- Resource temporarily unavailable
Jan 9 20:02:09 slavaya-omv proftpd[11049]: 192.168.254.112 (127.0.0.1[127.0.0.1]) - mod_tls.c: error initializing session: Отказано в доступе
Jan 9 20:02:45 slavaya-omv monit[10992]: 'proftpd' failed protocol test [FTP] at [localhost]:21 [TCP/IP] -- FTP: error receiving data -- Resource temporarily unavailable
Jan 9 20:02:45 slavaya-omv monit[10992]: 'proftpd' trying to restart
Jan 9 20:02:45 slavaya-omv monit[10992]: 'proftpd' stop: '/etc/init.d/proftpd stop'
Jan 9 20:02:45 slavaya-omv proftpd[11103]: 192.168.254.112 (127.0.0.1[127.0.0.1]) - mod_tls.c: error initializing session: Отказано в доступе
Jan 9 20:02:45 slavaya-omv systemd[1]: Stopping LSB: Starts ProFTPD daemon...
Jan 9 20:02:45 slavaya-omv proftpd[31802]: 192.168.254.112 - ProFTPD killed (signal 15)
Jan 9 20:02:45 slavaya-omv proftpd[31802]: 192.168.254.112 - ProFTPD 1.3.6 standalone mode SHUTDOWN
Jan 9 20:02:45 slavaya-omv proftpd[11121]: Stopping ftp server: proftpd.
Jan 9 20:02:45 slavaya-omv systemd[1]: proftpd.service: Succeeded.
Jan 9 20:02:45 slavaya-omv systemd[1]: Stopped LSB: Starts ProFTPD daemon.
Jan 9 20:02:45 slavaya-omv monit[10992]: 'proftpd' start: '/etc/init.d/proftpd restart'
Jan 9 20:02:45 slavaya-omv systemd[1]: Starting LSB: Starts ProFTPD daemon...
Jan 9 20:02:45 slavaya-omv proftpd[11150]: 192.168.254.112 - ProFTPD 1.3.6 (stable) (built Tue Mar 10 2020 23:03:08 UTC) standalone mode STARTUP
Jan 9 20:02:45 slavaya-omv proftpd[11140]: Starting ftp server: proftpd.
Jan 9 20:02:45 slavaya-omv systemd[1]: Started LSB: Starts ProFTPD daemon.
Jan 9 20:03:21 slavaya-omv monit[10992]: 'proftpd' failed protocol test [FTP] at [localhost]:21 [TCP/IP] -- FTP: error receiving data -- Resource temporarily unavailable
Jan 9 20:03:21 slavaya-omv monit[10992]: 'proftpd' trying to restart
Jan 9 20:03:21 slavaya-omv monit[10992]: 'proftpd' stop: '/etc/init.d/proftpd stop'
Jan 9 20:03:21 slavaya-omv proftpd[11196]: 192.168.254.112 (127.0.0.1[127.0.0.1]) - mod_tls.c: error initializing session: Отказано в доступе
Jan 9 20:03:21 slavaya-omv systemd[1]: Stopping LSB: Starts ProFTPD daemon...
Jan 9 20:03:21 slavaya-omv proftpd[11150]: 192.168.254.112 - ProFTPD killed (signal 15)
Jan 9 20:03:21 slavaya-omv proftpd[11150]: 192.168.254.112 - ProFTPD 1.3.6 standalone mode SHUTDOWN
Jan 9 20:03:21 slavaya-omv proftpd[11214]: Stopping ftp server: proftpd.
Jan 9 20:03:21 slavaya-omv systemd[1]: proftpd.service: Succeeded.
Jan 9 20:03:21 slavaya-omv systemd[1]: Stopped LSB: Starts ProFTPD daemon.
Jan 9 20:03:21 slavaya-omv monit[10992]: 'proftpd' start: '/etc/init.d/proftpd restart'
Jan 9 20:03:21 slavaya-omv systemd[1]: Starting LSB: Starts ProFTPD daemon...
Jan 9 20:03:21 slavaya-omv proftpd[11242]: 192.168.254.112 - ProFTPD 1.3.6 (stable) (built Tue Mar 10 2020 23:03:08 UTC) standalone mode STARTUP
Jan 9 20:03:21 slavaya-omv proftpd[11234]: Starting ftp server: proftpd.
Jan 9 20:03:21 slavaya-omv systemd[1]: Started LSB: Starts ProFTPD daemon.
Jan 9 20:03:57 slavaya-omv monit[10992]: 'proftpd' failed protocol test [FTP] at [localhost]:21 [TCP/IP] -- FTP: error receiving data -- Resource temporarily unavailable
Jan 9 20:03:57 slavaya-omv monit[10992]: 'proftpd' trying to restart
Jan 9 20:03:57 slavaya-omv monit[10992]: 'proftpd' stop: '/etc/init.d/proftpd stop'
Jan 9 20:03:57 slavaya-omv proftpd[11289]: 192.168.254.112 (127.0.0.1[127.0.0.1]) - mod_tls.c: error initializing session: Отказано в доступе
Jan 9 20:03:57 slavaya-omv systemd[1]: Stopping LSB: Starts ProFTPD daemon...
Jan 9 20:03:57 slavaya-omv proftpd[11242]: 192.168.254.112 - ProFTPD killed (signal 15)
Jan 9 20:03:57 slavaya-omv proftpd[11242]: 192.168.254.112 - ProFTPD 1.3.6 standalone mode SHUTDOWN
Jan 9 20:03:57 slavaya-omv proftpd[11307]: Stopping ftp server: proftpd.
Jan 9 20:03:57 slavaya-omv systemd[1]: proftpd.service: Succeeded.
Jan 9 20:03:57 slavaya-omv systemd[1]: Stopped LSB: Starts ProFTPD daemon.
Jan 9 20:03:57 slavaya-omv monit[10992]: 'proftpd' start: '/etc/init.d/proftpd restart'
Jan 9 20:03:57 slavaya-omv systemd[1]: Starting LSB: Starts ProFTPD daemon...
Jan 9 20:03:57 slavaya-omv proftpd[11334]: 192.168.254.112 - ProFTPD 1.3.6 (stable) (built Tue Mar 10 2020 23:03:08 UTC) standalone mode STARTUP
Jan 9 20:03:57 slavaya-omv proftpd[11326]: Starting ftp server: proftpd.
Jan 9 20:03:57 slavaya-omv systemd[1]: Started LSB: Starts ProFTPD daemon.
Jan 9 20:04:33 slavaya-omv monit[10992]: 'proftpd' failed protocol test [FTP] at [localhost]:21 [TCP/IP] -- FTP: error receiving data -- Resource temporarily unavailable
Jan 9 20:04:33 slavaya-omv monit[10992]: 'proftpd' trying to restart
Jan 9 20:04:33 slavaya-omv monit[10992]: 'proftpd' stop: '/etc/init.d/proftpd stop'
Jan 9 20:04:33 slavaya-omv proftpd[11381]: 192.168.254.112 (127.0.0.1[127.0.0.1]) - mod_tls.c: error initializing session: Отказано в доступе
Jan 9 20:04:33 slavaya-omv systemd[1]: Stopping LSB: Starts ProFTPD daemon.......
see attach - 10000 char limit