This is fixed with the organizer tracking in Kronolith H6.

User A creates a meeting "Meeting test" with two other atendees (C and 
D). All attendees in same Horde server.

D accepts meeting from Horde account, organizer receives an 
acceptation reply, all other attendees doesn't receives this reply.

C attendee uses a Thunderbird client whith Lightning plugin and it has 
C Horde calendar configured using CalDAV Subscription URL.

If C attendee accepts meeting from a Thunderbird client  then all 
other meeting participants receives a message notification sent from A 
account in Horde server: that's wrong.

The Accept url shown in Thunderbird client than A attendee is using 
its similar to this URL

https://hordeserver/horde/kronolith/attend.php?c=zTLuEgcMewfpGIe_lEtWAVE&e=MIu2Lo5IrVrMhbRY4kjUWfk&u=attendee.C@domain.es&a=accept

and the notify message sent to all other attendees shows that:

1. mail Subject says "C wants to notify you about changes in "Meeting test" "
2.  the included ICS file shows that C attendee is the organizer of 
this meeting => thats wrong, the original organizer is A.

I've attached the invitation ICS file example sent with this 
notification, in this example usuari.ana_at_upcnet.es was the original 
organizer, but when I (albert.solana_at_upcnet.es) accepted the 
meeting from a Thunberbird client I appears as organizer in this ICS 
file sent to all other attendees.