The Fail2Ban service fails to start on a Plesk server: ModuleNotFoundError: No module named 'fail2ban'

Symptoms

• The fail2ban service on the server fails to start and the systemctl status fail2ban command output is similar to the following:
  

  # [root@server ~]# systemctl status fail2ban
  ● fail2ban.service - Fail2Ban Service
  Loaded: loaded (/usr/lib/systemd/system/fail2ban.service; enabled; vendor preset: disabled)
  Drop-In: /usr/lib/systemd/system/fail2ban.service.d
  └─50-ensure-log.conf
  /etc/systemd/system/fail2ban.service.d
  └─override.conf
  Active: activating (auto-restart) (Result: exit-code) since Wed 2025-10-29 14:43:10 UTC; 1s ago
  Docs: man:fail2ban(1)
  Process: 1866826 ExecStart=/usr/bin/fail2ban-server -xf start (code=exited, status=1/FAILURE)
  Process: 1866822 ExecStartPre=/bin/sh -c umask 077; touch /var/log/fail2ban.log; restorecon /var/log/fail2ban.log >/dev/null 2>&1 (code=exited, status=0/SUCCESS)
  Process: 1866820 ExecStartPre=/bin/mkdir -p /run/fail2ban (code=exited, status=0/SUCCESS)
  Main PID: 1866826 (code=exited, status=1/FAILURE)

• Running the journalctl -ufail2ban command produces output that is similar to the following:

  CONFIG_TEXT: Oct 29 12:58:20 server.example.com systemd[1]: Started Fail2Ban Service.
  Oct 29 12:58:20 server.example.com fail2ban-server[1783605]: Traceback (most recent call last):
  Oct 29 12:58:20 server.example.com fail2ban-server[1783605]: File "/usr/bin/fail2ban-server", line 34, in <module>
  Oct 29 12:58:20 server.example.com fail2ban-server[1783605]: from fail2ban.client.fail2banserver import exec_command_line, sys
  Oct 29 12:58:20 server.example.com fail2ban-server[1783605]: ModuleNotFoundError: No module named 'fail2ban'
  Oct 29 12:58:20 server.example.com systemd[1]: fail2ban.service: Main process exited, code=exited, status=1/FAILURE
  Oct 29 12:58:20 server.example.com systemd[1]: fail2ban.service: Failed with result 'exit-code'.
  Oct 29 12:58:20 server.example.com systemd[1]: fail2ban.service: Service RestartSec=100ms expired, scheduling restart.
  Oct 29 12:58:20 server.example.com systemd[1]: fail2ban.service: Scheduled restart job, restart counter is at 5.
  Oct 29 12:58:20 server.example.com systemd[1]: Stopped Fail2Ban Service.
  Oct 29 12:58:20 server.example.com systemd[1]: fail2ban.service: Start request repeated too quickly.
  Oct 29 12:58:20 server.example.com systemd[1]: fail2ban.service: Failed with result 'exit-code'.
  Oct 29 12:58:20 server.example.com systemd[1]: Failed to start Fail2Ban Service.

• Error that are similar to the following appear during an attempt to access Tools & Settings > IP Address Banning (Fail2Ban) > Banned IP Addresses menu:

  PLESK_ERROR: Server Error
  500 PleskUtilException
  f2bmng failed: Traceback (most recent call last): File "/usr/bin/fail2ban-client", line 34, in <module> from fail2ban.client.fail2banclient import exec_command_line, sys ModuleNotFoundError: No module named 'fail2ban' ERROR:__main__:Command '['/usr/bin/fail2ban-client', '--str2sec', '1w']' returned non-zero exit status 1. ERROR:__main__:Not a time string: 1w
  Type PleskUtilException
  Message f2bmng failed: Traceback (most recent call last): File "/usr/bin/fail2ban-client", line 34, in <module> from fail2ban.client.fail2banclient import exec_command_line, sys ModuleNotFoundError: No module named 'fail2ban' ERROR:__main__:Command '['/usr/bin/fail2ban-client', '--str2sec', '1w']' returned non-zero exit status 1. ERROR:__main__:Not a time string: 1w
  File Agent.php
  Line 159

Cause

The Fail2Ban component in Plesk can only work when /usr/bin/python3 leads to Python 3.6, however on this Linux server, /usr/bin/python3 is linked to Python 3.9 (or some other newer version) instead of Python 3.6:

# /usr/bin/python3 -V
Python 3.9.0

In general, what this shows is that /usr/bin/python3 on this server was most likely altered manually by the server's system administrator for a specific reason.

With that said, the general situation of a failed Fail2Ban service to which this leads is also considered a Plesk bug with ID PPPM-14555.

You may track the progress in relation to this bug in the Change Log for Plesk Obsidian

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Resolution

Until the bug is resolved permanently, as a workaround, you may modify the Fail2Ban binary files in a way that would make them use Python 3.6 by following these steps:

1. Log into the server via SSH

2. Modify the first line of the /usr/bin/fail2ban-client and /usr/bin/fail2ban-server files with your favorite command line text editor and change it from #!/usr/bin/python3 to #!/usr/bin/python3.6

3. Restart the fail2ban service:
   

   # systemctl restart fail2ban

Note: As an alternative to using command-line text editors, you may run the following commands in order to modify the mentioned files:

# sed -i '1s|^#!/usr/bin/python3|#!/usr/bin/python3.6|' /usr/bin/fail2ban-client
# sed -i '1s|^#!/usr/bin/python3|#!/usr/bin/python3.6|' /usr/bin/fail2ban-server