CentOS 7
rpm -qv pykolab
pykolab-0.8.3-3.3.el7.kolab_16.noarch`

If I import an Event-File (ics) from OpenXchange, not every rrule is correct interpreted from roundcube. The Rule "BYSETPOS=-1" for a event every last weekday, will not be interpreted.

Here my imported file:

BEGIN:VCALENDAR
PRODID:Open-Xchange
VERSION:2.0
CALSCALE:GREGORIAN
BEGIN:VEVENT
DTSTAMP:20161005T104702Z
SUMMARY:Grillabend
DTSTART;TZID=Europe/Berlin:20160629T190000
DTEND;TZID=Europe/Berlin:20160630T020000
CLASS:PRIVATE
LOCATION:Street 1
TRANSP:OPAQUE
CATEGORIES:privat
RRULE:FREQ=MONTHLY;INTERVAL=1;BYDAY=WE;BYSETPOS=-1
UID:8ff125c3-2300-4659-9f70-fa56fc7646d2
CREATED:20160929T072209Z
LAST-MODIFIED:20160929T072230Z
ORGANIZER:mailto:andreas@example.org
SEQUENCE:0
ATTENDEE;ROLE=REQ-PARTICIPANT;CUTYPE=INDIVIDUAL;CN="Andreas";PARTSTAT=ACCEPTED:mailto:andreas@example.org
END:VEVENT
END:VCALENDAR

The BYSETPOS is not interpreted correctly, so I see the event every Wednesday in roundcube.

Here a exported file from kolab, where I created the same event via gui.

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-Roundcube libcalendaring 1.2-gitSabreSabre VObject 3.5.2EN
CALSCALE:GREGORIAN
BEGIN:VEVENT
UID:8ff125c3-2300-4659-9f70-fa56fc7646d2
DTSTAMP:20161005T125244Z
DTSTAMP:20161005T105244Z
CREATED:20161005T104810Z
LAST-MODIFIED:20161005T105224Z
DTSTART;TZID=Europe/Berlin:20160629T190000
DTEND;TZID=Europe/Berlin:20160630T020000
SUMMARY:Grillabend
LOCATION:Street 1
SEQUENCE:1
RRULE:FREQ=MONTHLY;INTERVAL=1;BYDAY=-1WE
CATEGORIES:privat
TRANSP:OPAQUE
CLASS:PRIVATE
ORGANIZER:mailto:andreas@example.org
END:VEVENT
END:VCALENDAR