sages
You do not say what time the backup is scheduled to start & whether it is already running & creating backup files at the time the error message occurs (ie nfs not responding).
The fail2ban event is at 1.40 whereas the not responding nfs is at 2.00, 20 mins later, so I do not see them related
If you think fail2ban is involved, then disable fail2ban & see if the backup runs OK then.
Any access attemps being made that fail2ban identifies & bans, are still prevented from gaining access by SME server, so it is still quite safe to run without fail2ban enabled.
Feb 28 02:00:22 fwbox kernel: nfs: server 192.168.128.10 not responding, still trying
Feb 28 02:24:00 fwbox kernel: INFO: task dar:23036 blocked for more than 120 seconds.