If fail2ban parses a log (e.g. pure-ftpd) where the hostname of the attacker is enclosed in brackets like "(hostname.domain.tld)", it fails. In the fail2ban.log, you can see the following error:
2009-07-16 07:51:15,598 fail2ban.filter : WARNING Unable to find a corresponding IP address for ns10.hostinglmi.net)
It seems, that the closing bracket is kept as the hostname, which indeed fails.
I checked this on ubuntu 8.04 with a pure-ftpd filter which was already installed. I found occurrences in the net where this error also happens on apache logs.
I fixed it in the filter.py by replacing the ")" with "". But I guess it's possible to fix it with a change of the <HOST> substitution regex.