Of course - and thanks for your great work!

Restarted fail2ban in DEBUG mode:

root@newmarlin:~# tail -f /var/log/fail2ban.log
2025-04-27T07:12:34.968427+00:00 newmarlin fail2ban.utils[28595]: ERROR ffffacebd340 -- stderr: '2025-04-26T20:04:26.740069+00:00 newmarlin sshd[12735]: Invalid user solx from SOMEIMP port 59272'
2025-04-27T07:12:34.968494+00:00 newmarlin fail2ban.utils[28595]: ERROR ffffacebd340 -- stderr: '2025-04-27T05:51:07.346446+00:00 newmarlin sshd[26187]: Invalid user ubuntu from *SOMEIP*  port 39368'
2025-04-27T07:12:34.968636+00:00 newmarlin fail2ban.utils[28595]: ERROR ffffacebd340 -- stderr: '2025-04-27T06:01:32.901474+00:00 newmarlin sshd[26458]: Invalid user validator from *SOMEIP*  port 39306'
2025-04-27T07:12:34.968701+00:00 newmarlin fail2ban.utils[28595]: ERROR ffffacebd340 -- stderr: '2025-04-27T06:12:14.910224+00:00 newmarlin sshd[26683]: Invalid user node from *SOMEIP*  port 39082'
2025-04-27T07:12:34.968757+00:00 newmarlin fail2ban.utils[28595]: ERROR ffffacebd340 -- stderr: '2025-04-27T06:22:53.212625+00:00 newmarlin sshd[26851]: Invalid user solana from *SOMEIP*  port 38860'
2025-04-27T07:12:34.968833+00:00 newmarlin fail2ban.utils[28595]: ERROR ffffacebd340 -- stderr: '2025-04-27T06:33:28.101646+00:00 newmarlin sshd[27406]: Invalid user sol from *SOMEIP*  port 38644: 2: /usr/local/bin/fail2ban_banned_db: not found'
2025-04-27T07:12:34.968896+00:00 newmarlin fail2ban.utils[28595]: ERROR ffffacebd340 -- returned 127
2025-04-27T07:12:34.968969+00:00 newmarlin fail2ban.utils[28595]: INFO HINT on 127: "Command not found".  Make sure that all commands in ['f2bV_ipjailmatches=$0 \n/usr/local/bin/fail2ban_banned_db ban *SOMEIP*  22 tcp sshd 15 $f2bV_ipjailmatches', '2025-04-26T19:22:13.657215+00:00 newmarlin sshd[11957]: Invalid user user from *SOMEIP*  port 44250\n2025-04-26T19:32:30.810450+00:00 newmarlin sshd[12107]: Invalid user vali from *SOMEIP*  port 40948\n2025-04-26T19:42:55.741753+00:00 newmarlin sshd[12315]: Invalid user ubuntu from *SOMEIP*  port 37646\n2025-04-26T19:53:35.590692+00:00 newmarlin sshd[12501]: Invalid user ada from *SOMEIP*  port 34344\n2025-04-26T20:04:26.740069+00:00 newmarlin sshd[12735]: Invalid user solx from *SOMEIP*  port 59272\n2025-04-27T05:51:07.346446+00:00 newmarlin sshd[26187]: Invalid user ubuntu from *SOMEIP*  port 39368\n2025-04-27T06:01:32.901474+00:00 newmarlin sshd[26458]: Invalid user validator from *SOMEIP*  port 39306\n2025-04-27T06:12:14.910224+00:00 newmarlin sshd[26683]: Invalid user node from *SOMEIP*  port 39082\n2025-04-27T06:22:53.212625+00:00 newmarlin sshd[26851]: Invalid user solana from *SOMEIP*  port 38860\n2025-04-27T06:33:28.101646+00:00 newmarlin sshd[27406]: Invalid user sol from *SOMEIP*  port 38644'] are in the PATH of fail2ban-server process (grep -a PATH= /proc/`pidof -x fail2ban-server`/environ). You may want to start "fail2ban-server -f" separately, initiate it with "fail2ban-client reload" in another shell session and observe if additional informative error messages appear in the terminals.
2025-04-27T07:12:34.969869+00:00 newmarlin fail2ban.actions[28595]: ERROR Failed to execute ban jail 'sshd' action 'banned_db' info 'ActionInfo({'ip': '*SOMEIP* ', 'family': 'inet4', 'fid': <function Actions.ActionInfo.<lambda> at 0xffffaf83a020>, 'ipjailmatches': '2025-04-26T19:22:13.657215+00:00 newmarlin sshd[11957]: Invalid user user from *SOMEIP*  port 44250\n2025-04-26T19:32:30.810450+00:00 newmarlin sshd[12107]: Invalid user vali from *SOMEIP*  port 40948\n2025-04-26T19:42:55.741753+00:00 newmarlin sshd[12315]: Invalid user ubuntu from *SOMEIP*  port 37646\n2025-04-26T19:53:35.590692+00:00 newmarlin sshd[12501]: Invalid user ada from *SOMEIP*  port 34344\n2025-04-26T20:04:26.740069+00:00 newmarlin sshd[12735]: Invalid user solx from *SOMEIP*  port 59272\n2025-04-27T05:51:07.346446+00:00 newmarlin sshd[26187]: Invalid user ubuntu from *SOMEIP*  port 39368\n2025-04-27T06:01:32.901474+00:00 newmarlin sshd[26458]: Invalid user validator from *SOMEIP*  port 39306\n2025-04-27T06:12:14.910224+00:00 newmarlin sshd[26683]: Invalid user node from *SOMEIP*  port 39082\n2025-04-27T06:22:53.212625+00:00 newmarlin sshd[26851]: Invalid user solana from *SOMEIP*  port 38860\n2025-04-27T06:33:28.101646+00:00 newmarlin sshd[27406]: Invalid user sol from *SOMEIP*  port 38644', 'ipjailfailures': 15, 'raw-ticket': <function Actions.ActionInfo.<lambda> at 0xffffaf83a7a0>})': Error banning *SOMEIP* #012Traceback (most recent call last):#012  File "/usr/lib/python3/dist-packages/fail2ban/server/actions.py", line 513, in __checkBan#012    action.ban(aInfo)#012  File "/usr/lib/python3/dist-packages/fail2ban/server/action.py", line 568, in ban#012    raise RuntimeError("Error banning %(ip)s" % aInfo)#012RuntimeError: Error banning *SOMEIP*
2025-04-27T07:12:34.970108+00:00 newmarlin fail2ban.actions[28595]: DEBUG Banned 6 / 6, 6 ticket(s) in 'sshd'

It's not clear what the problem is according to pasted log lines.
Did you try to restart fail2ban service?