Fail2ban : ERROR No ‘host’ group in ‘kernel

If this happened while you were installing the shrunk window jail in Fail2Ban,

nano /etc/fail2ban/filter.d/shrunk-window.conf

Replace

failregex = TCP\: Peer \:.* unexpectedly shrunk window.*repaired+

With :

failregex = TCP\: Peer \:.* unexpectedly shrunk window.*repaired+

The was missing.