[horde] Problem accessing calDAV shared calendars

[horde at adhelios.fr]

Hello,
I have a problem :

We have 5 users, all calendars are shared.
So each one can see/edit/modify/delete events on any other calendars  
using horde webmail, as expected.

Calendar sharing also works in thunderbird/lightning with ICS calendars :
We added the 5 .ics of each users on each of their computers,
and each can see/edit/modify/delete events of the others.
But .ics is very long to load so we want to use 5 CalDAV instead of  
ICS in thunderbird/lightning.

Problem is, only the calDAV owned by the user connecting with  
Lightning can be seen and modified :
the CalDAV of the 4 other users never load.

When I copy/paste the CalDAV url of one user in Mozilla Firefox or  
Chrome and specify another username and password than the owner :
"Une erreur fatale s'est produite
Call to a member function toHash() on a non-object
Les détails ont été consignés pour l'administrateur."

In var/log/messages we have only this error :
HORDE:[kronolith] Call to a member function toHash() on a non-object  
[pid 4865 on line 686 of  
"/var/www/html/mail/horde/kronolith/lib/Application.php"]

Somebody know what is wrong and what i must do
Does anybody know what is wrong ? Is there a way to get my 5 shared  
calDAV to work in Thunderbird/lightning ?


Thanks for your help.
GG

//
Horde server configuration :
OS : CENTOS 7
Horde Groupware Webmail Edition Horde Groupware Webmail Edition 5.2.11
Agenda Agenda (kronolith) 4.2.11
Carnet d'adresses Carnet d'adresses (turba) 4.2.11
content 2.0.5
Courrier Courrier (imp) 6.2.11
Filtres Filtres (ingo) 3.2.7
Gestionnaire de fichiers Gestionnaire de fichiers (gollem) 3.0.6
Horde Horde (horde) 5.2.8
Notes Notes (mnemo) 4.2.8
Signets Signets (trean) 1.1.3
Tâches Tâches (nag) 4.2.6
timeobjects 2.1.0
Horde_ActiveSync
Horde_Alarm
Horde_Auth
Horde_Cache
Horde_Core
Horde_Dav
Horde_Group
Horde_History.
Horde_Imap_Client
Horde_Lock
Horde_Perms
Horde_Prefs
Horde_Queue
Horde_SessionHandler
Horde_SyncMl
Horde_Token
Horde_Vfs

Mail/calendar clients configurations :
WINDOWS 7, 8.1 PRO
Thunderbird 38.3.0 / lightning 4.0.3.1