I've tried again and now there's no issues, it's all ok, I haven't 
been able to reproduce the same problem.

My apologies.

I can't reproduce this with the most recent stable version. Please 
update and try again.

This is what I'm doing:

* Creating a new recurring appointment that occurs 10 times and 
inviting two attendees who are also Horde users.

* The attendees successfully accept this event and I receive the 
notification that they are attending.

* I delete one of instances and notify attendees.

* All attendees receive the notification and are able to update their 
copy of the event.

* I delete another instance of the series, and notify attendees. My 
copy of the event successfully shows the second instance is removed.

* Attendees receive the notification and are still able to update 
their copy of the event.

I can repeat the above numerous times with the exceptions being 
created correctly each time.

My server is configured with Kronolith 4.2.11 and  Horde 5.2.8.

I've noticed problems in Horde webmail when I try to delete specific 
entries in a recurrent event serie.

For instance I created a recurrent event serie (10 saturdays) with two 
attendees.

I can delete an specific event and notify all attendees.
Attendees receive e-mail notification and can update their calendars 
where this event is removed.

But if I try to delete another specific event  and notify all 
attendees then this one is not removed from my calendar, the attendees 
receives the e-mail notification but when they accept the event is not 
removed from their calendars. No red error message pop-up appears in 
all of those actions.

Sometimes I've noticed that after logout and login I can delete this 
specific event and attendees receives notification and they can delete 
it, but next attempts usually fail and sometimes logout/login "trick" 
neither works.

I've noticed that removing an especific entry from recurring event   
from Thunderbird 3.8.7.2 Lighning 4.0.5.2 always seems to work fine: 
It's deleted automatically from my calendar and attendees receives 
email notification and can delete it without problems. I've noticed 
that it removes the events which deletion failed before in Horde 
website, the ics file contains all exceptions (specific event deletes):

- Example 1, notification sent by Thunderbird:
UserA  wants to notify you about changes in "repetida setmanal ds".
Start:         14/05/16 0:00
End:         14/05/16 0:59
Summary:         repetida setmanal ds
Recurrence:         Weekly: Recurs every 1 week(s) on: Saturday
Ends after: 10 times
Exceptions on: 18/06/16 06/11/16 28/05/16 14/05/16 25/06/16 07/02/16 02/07/16

If I compare with what is notified from Kronolith in Horde webmail I just see:

- Example 2, delete 25/06/16  ics file generated:
albert.solana has cancelled an instance of the recurring "repetida 
setmanal ds".
Start:         25/06/16 0:00
End:         25/06/16 0:59
Summary:         repetida setmanal ds
Recurrence:         Weekly: Recurs every 1 week(s) on: Saturday
Ends after: 10 times
Attendees:
Name         Role         Status
Usuari Elena2         Required Participant         Awaiting Response
albert.solana@upcnet.es         Required Participant         Awaiting Response
Usuari Ana         Required Participant         Awaiting Response
Exceptions:
Start:         25/06/16 0:00
End:         25/06/16 1:00
Summary:         repetida setmanal ds


- Example 3, detele 07/02/16 ics file generated:

albert.solana has cancelled an instance of the recurring "repetida 
setmanal ds".
Start:          0:07/02/1600
End:         07/02/16 0:59
Summary:         repetida setmanal ds
Recurrence:         Weekly: Recurs every 1 week(s) on: Saturday
Ends after: 10 times


I've upload a zip file with these 3 ics file examples.