[horde] Class 'Sabre\DAVACL\PrincipalBackend\AbstractBackend' not found

[Dominik Erdmann]

Hi!

I upgraded my Git-installation to latest git-development. I noticed that 
DAV (CardDAV, CalDAV) does not work. I get the following message:

[horde] Class 'Sabre\DAVACL\PrincipalBackend\AbstractBackend' not found 
[pid 20898 on line 29 of 
"/<path_to>/horde-base/Dav/lib/Horde/Dav/Principals.php"]

Don't why this does not work any more. All config files seems to be 
identical.
Could this be an issue with having the installation not on root-level of 
domain?

Hint1: I configured in /<path_to>/horde-base/base/config/registry.local.php
$app_fileroot = '/var/www/domain.com/horde';
$app_webroot = '/horde';

Hint2: I updated from
$apps = array('content', 'gollem', 'imp', 'ingo', 'kronolith', 'mnemo', 
'nag', 'passwd', 'timeobjects', 'trean', 'turba');
to
$conf['apps'] = array();

Thanks
Dominik