I have noticed exceptions to recurring events being filtered out when using the roundcube interface (or iRony) but not when using an Android EAS client via syncroton. I have just set up a new test server (CentOS 7, Kolab 16 repo) today.

An example VEVENT is included below:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-Roundcube libcalendaring 1.2.3SabreSabre VObject 3.5.3EN
CALSCALE:GREGORIAN
BEGIN:VEVENT
UID:3BBC-57D66800-11-5E486100
DTSTAMP:20170101T214959Z
CREATED:20160912T083358Z
LAST-MODIFIED:20170101T202915Z
DTSTART;TZID=Europe/Berlin:20160907T083000
DTEND;TZID=Europe/Berlin:20160907T123000
SUMMARY:Test
LOCATION:Here nor there
SEQUENCE:6
RRULE:FREQ=WEEKLY;INTERVAL=1;UNTIL=20170714
EXDATE:20161026T063000Z
EXDATE:20161228T073000Z
EXDATE:20170104T073000Z
EXDATE:20170301T073000Z
EXDATE:20170426T063000Z
EXDATE:20170503T063000Z
TRANSP:OPAQUE
CLASS:PUBLIC
ORGANIZER;CN="Tester, Test":mailto:test.tester@test.lan
END:VEVENT
BEGIN:VTIMEZONE
TZID:Europe/Berlin
BEGIN:STANDARD
DTSTART:20151025T010000
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
TZNAME:CET
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:20160327T010000
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
TZNAME:CEST
END:DAYLIGHT
BEGIN:STANDARD
DTSTART:20161030T010000
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
TZNAME:CET
END:STANDARD
END:VTIMEZONE
END:VCALENDAR