fail2ban restarts frequently after plesk 12.5.30 update on ubuntu 12.04

[PeterKi]

I was happily running a vServer with plesk on ubuntu 12.04 LTS.
2 days ago I upgraded plesk from 12.0 to 12.5.30.
Since then I see frequent restarts of the fail2ban service (around every other 4 hours)
/var/log/fail2ban.log shows errors from iptables like:
ERROR iptables -D INPUT -p tcp -j f2b-default
iiptables v1.4.12: Couldn't load target `f2b-default':No such file or directory\n
ERROR Failed to stop jail 'plesk-proftpd' action

It seems that the update broke something but I don't know how to troubleshoot iit.