Thank you very much, the patch did the trick for me!

IMP 6.2.6
Horde 5.2.3
Kronolith 4.2.4

Hm, I still have the issue:
IMP 7.0.0
Horde 5.3.0
Kronolith 4.3.0

*Milestone* 6.2.7

If you apply this patch [1] in 6.2.6 it work.

[1]https://github.com/horde/horde/commit/d118cf6ded68fa4317da765e67e9f4d3f9c490bd

Hi,

I still see this problem in IMP 6.2.6.  I am too when clicking on the 
pencil in Calendar I am getting "Admin access not enabled" and my OS/X 
Copntacts and Calendar failing with Authentication Error. After 
reverting back to IMP 6.2.4 all works fine.

Regards,
Horst

Changes have been made in Git (FRAMEWORK_5_2):

commit 582fe21c6d1079ac52f909e139952c2deb315c88
Author: Jan Schneider <jan@horde.org>
Date:   Thu Jan 22 12:23:07 2015 +0100

     [jan] Fix regression with not being able to list users (Bug #13808).

  imp/docs/CHANGES |    1 +
  imp/package.xml  |    2 ++
  2 files changed, 3 insertions(+), 0 deletions(-)

http://github.com/horde/horde/commit/582fe21c6d1079ac52f909e139952c2deb315c88

Changes have been made in Git (FRAMEWORK_5_2):

commit d118cf6ded68fa4317da765e67e9f4d3f9c490bd
Author: Jan Schneider <jan@horde.org>
Date:   Thu Jan 22 12:21:27 2015 +0100

     Revert "Move disabling of auth methods to init phase."

     This reverts commit 0a57efa0b717cdd292102d8e3b07644ada17bd17.

     Obviously this broke more cases than it fixed.

     Bug: 13808

  imp/lib/Application.php |   14 +++++++-------
  1 files changed, 7 insertions(+), 7 deletions(-)

http://github.com/horde/horde/commit/d118cf6ded68fa4317da765e67e9f4d3f9c490bd

Same problem here, and:

<d:error><s:exception>Sabre\DAV\Exception\NotFound</s:exception><s:message>User USERNAME does not 
exist</s:message><s:sabredav-version>1.8.10</s:sabredav-version></d:error>

and the adressbook and calendar sync doesn't work after update.

Hi all,

can confirm this bug too. Occurs when trying to edit calendar within 
Horde and also impossible to access calendar via CalDAV from 
Thunderbird.

Regards
Torsten

https://github.com/horde/horde/commit/0a57efa0b717cdd292102d8e3b07644ada17bd17

Can confirm this bug.

Looks like IMPs list capability isn't removed soon enough as DAV trys 
to call IMP_Application::authUserList even though the backend doesn't 
support it:

exception 'IMP_Exception' with message 'Admin access not enabled.' in 
/var/www/horde/horde/imp/lib/Factory/AuthImap.php:37
Stack trace:
#0 /var/www/horde/pear/php/Horde/Injector/Binder/Factory.php(119): 
IMP_Factory_AuthImap->create(Object(Horde_Injector))
#1 /var/www/horde/pear/php/Horde/Injector.php(238): 
Horde_Injector_Binder_Factory->create(Object(Horde_Injector))
#2 /var/www/horde/pear/php/Horde/Injector.php(272): 
Horde_Injector->createInstance('IMP_AuthImap')
#3 /var/www/horde/horde/imp/lib/Application.php(410): 
Horde_Injector->getInstance('IMP_AuthImap')
#4 [internal function]: IMP_Application->authUserList()
#5 /var/www/horde/pear/php/Horde/Registry.php(1191): 
call_user_func_array(Array, Array)
#6 /var/www/horde/pear/php/Horde/Core/Auth/Application.php(323): 
Horde_Registry->callAppMethod('imp', 'authUserList')
#7 /var/www/horde/pear/php/Horde/Auth/Base.php(433): 
Horde_Core_Auth_Application->listUsers()
#8 /var/www/horde/pear/php/Horde/Core/Auth/Application.php(342): 
Horde_Auth_Base->exists('ud_1p15')
#9 /var/www/horde/pear/php/Horde/Core/Auth/Application.php(337): 
Horde_Core_Auth_Application->exists('ud_1p15')
#10 /var/www/horde/pear/php/Horde/Core/Auth/UsernameHook.php(138): 
Horde_Core_Auth_Application->exists('ud_1p15')
#11 /var/www/horde/pear/php/Horde/Dav/Principals.php(90): 
Horde_Core_Auth_UsernameHook->exists('ud_1p15')
#12 
/var/www/horde/pear/php/Sabre/DAVACL/AbstractPrincipalCollection.php(115): 
Horde_Dav_Principals->getPrincipalByPath('principals/ud_1...')
#13 /var/www/horde/pear/php/Sabre/DAV/Collection.php(54): 
Sabre\DAVACL\AbstractPrincipalCollection->getChild('ud_1p15')
#14 /var/www/horde/pear/php/Sabre/DAV/ObjectTree.php(98): 
Sabre\DAV\Collection->childExists('ud_1p15')
#15 /var/www/horde/pear/php/Sabre/DAVACL/Plugin.php(701): 
Sabre\DAV\ObjectTree->nodeExists('principals/ud_1...')
#16 [internal function]: Sabre\DAVACL\Plugin->beforeMethod('GET', 
'principals/ud_1...')
#17 /var/www/horde/pear/php/Sabre/DAV/Server.php(433): 
call_user_func_array(Array, Array)
#18 /var/www/horde/pear/php/Sabre/DAV/Server.php(455): 
Sabre\DAV\Server->broadcastEvent('beforeMethod', Array)
#19 /var/www/horde/pear/php/Sabre/DAV/Server.php(214): 
Sabre\DAV\Server->invokeMethod('GET', 'principals/ud_1...')
#20 /var/www/horde/pear/php/Horde/Rpc/Webdav.php(66): Sabre\DAV\Server->exec()
#21 /var/www/horde/horde/rpc.php(159): Horde_Rpc_Webdav->getResponse('')
#22 {main}

With latest IMP 6.2.5 I can no longer sync my Thunderbird with 
Lightning and SoGo Connector against my Horde Groupware.

When I try to access e.g. 
https://myhorde/rpc.php/addressbooks/steffen@hauihau.de/contacts:steffen@hauihau.de/ I'm getting a UserNotFound Exception from SabreDav (same with accessing the kronolith calendar via CalDAV). I'm also not able to edit the settings of a calendar, when I click on the pencil I get an error saying "Admin access not enabled.". As it was working with IMP 6.2.4, I digged a bit deeper and was able to break it down to commit 0a57efa0b717cdd292102d8e3b07644ada17bd17. Reverting it fixes the issues for 
me.

If you need further information, please let me know.