I missed that the original ticket was for H3.  This fix will not be 
back ported to Horde 3.  Event recurrence and exception handling has 
been rewritten for Horde 4.

But this is not qualified for Horde Groupware Webmail Edition 1.2.6, is it?

E.g. kronolith/lib/Api.php doesn't exist, though api.php (in small letters).

See if this helps...

Changes have been made in Git for this ticket:

More complete support for reccuring event exceptions when dealing with 
iCalendar files Bug: 9091

http://git.horde.org/diff.php/framework/iCalendar/iCalendar.php?rt=horde-git&r1=767847624b436609597b50aa7aa372470cd8fb65&r2=3bd6cf612aa29fde902bb6a88b18b548b3aaa14b
http://git.horde.org/diff.php/kronolith/lib/Api.php?rt=horde-git&r1=dd51814c9e217aafa20b3660a98050f64a6e92b9&r2=3bd6cf612aa29fde902bb6a88b18b548b3aaa14b
http://git.horde.org/diff.php/kronolith/lib/Event.php?rt=horde-git&r1=5e9d5cbaf49d9e290c2111a99fe4444066c23e3e&r2=3bd6cf612aa29fde902bb6a88b18b548b3aaa14b
http://git.horde.org/diff.php/kronolith/lib/Kronolith.php?rt=horde-git&r1=0e5db4432a7cd1e122b0d5a17bc083c4d9baf624&r2=3bd6cf612aa29fde902bb6a88b18b548b3aaa14b
http://git.horde.org/diff.php/kronolith/lib/View/ExportEvent.php?rt=horde-git&r1=a700826d0e39e7c64a82f4fcf82e11e3ac0e7e07&r2=3bd6cf612aa29fde902bb6a88b18b548b3aaa14b

We only support exceptions through EXDATE properties at the moment. 
And since both vEvent objects share the same UID, only one is used, 
which ever comes last.
Parent-child-relationships for recurring events and exception events 
have only been added recently to Kronolith 3. Converting this 
relationship from and to iCalendar objects has yet to be implemented.

I tried to sync an kalendar with lightning. I discovered exceptions 
from recurring events are not synchronized correctly (see 
http://article.gmane.org/gmane.comp.horde.kronolith/4988/)

Now I made a sniff:

---------------------original Event created in Kronolith (downloaded 
as iCal via rpc.php):
BEGIN:VEVENT
DTSTART:20100607T100000Z
DTEND:20100607T110000Z
DTSTAMP:20100611T070016Z
UID:20100611085912.82426gca3zlvh6o0@groupware.zew.uni-hannover.de
CREATED:20100611T065912Z
LAST-MODIFIED:20100611T065912Z
SUMMARY:serientermin
ORGANIZER;CN="Michael Gröne":mailto:michael.groene@zew.uni-hannover.de
CLASS:PUBLIC
STATUS:CONFIRMED
TRANSP:OPAQUE
RRULE:FREQ=DAILY;INTERVAL=1;UNTIL=20100612T215959Z
END:VEVENT
END:VCALENDAR
---------------------------------------------


---------------------now changed Event in Lightning (tuesdays exception)
-- this is a network-sniff by wireshark! --
BEGIN:VEVENT
CREATED:20100611T065912Z
LAST-MODIFIED:20100611T074043Z
DTSTAMP:20100611T074043Z
UID:20100611085912.82426gca3zlvh6o0@groupware.zew.uni-hannover.de
SUMMARY:serientermin
STATUS:CONFIRMED
ORGANIZER;CN=Michael Grne:mailto:michael.groene@zew.uni-hannover.de
RRULE:FREQ=DAILY;UNTIL=20100611T215959Z
DTSTART:20100607T100000Z
DTEND:20100607T110000Z
CLASS:PUBLIC
TRANSP:OPAQUE
X-MOZ-GENERATION:1
END:VEVENT
BEGIN:VEVENT
CREATED:20100611T073739Z
LAST-MODIFIED:20100611T074043Z
DTSTAMP:20100611T074043Z
UID:20100611085912.82426gca3zlvh6o0@groupware.zew.uni-hannover.de
SUMMARY:serientermin aunahme am dienstag (beginnt heute erst um 14h)
STATUS:CONFIRMED
RECURRENCE-ID:20100608T100000Z
ORGANIZER;CN=Michael Grne:mailto:michael.groene@zew.uni-hannover.de
DTSTART:20100608T120000Z
DTEND:20100608T130000Z
TRANSP:OPAQUE
SEQUENCE:1
CLASS:PUBLIC
X-MOZ-GENERATION:1
END:VEVENT
END:VCALENDAR
--------------------------------------

result is: in kronolith there are no changes and after reloading the 
calendar in lightning, the exception will flush.


Now I tried the following:
Importing the iCal manually, Lightning tried to send to the webserver. 
The results here become still stranger.
In Kronolith the exception will be displayed, but the belonging 
recurring event ("serientermin") is not visible, any more.
In Lightning instead (after reloading) the recurring event is 
displayed, but the exception not.