Hi Jochen,
I had the exact same issue and resolved it. The error message actually hints to the first problem already. After installing fail2ban on a Raspberry Pi (B for me), the jail.conf has invalid settings. The first issue is with a double definition of [port] in the [pam-generic] section. It once says port = all and once port = anyport. I deleted the latter. A second issue is that in [DEFAULT] the action parameter is defined twice, whereas the second definition just states action = %(action_)s. I commented the that second definition out. Then I restarted fail2ban and it seems to work since then (at least it starts up and correctly logs wrong login attempts - so all it should do).
I hope this helps!
Btw, I followed https://github.com/owncloud/documentation/issues/3078 to secure owncloud with fail2ban (see instruction from dercorn further below)