Take a look at imp/config/backends.php too.

I also don't know what  "in IMP allow user listing?" means
I found no option in imp config

I have the same problem .. backend Auth is also IMP

there is a patch on http://bugs.horde.org/ticket/12307
but it doesn't seem to work
after patch no error if i open it over the browser ... but dav is emtpy

I have checked the backends.php and the imp configuration page and 
haven´t found any settings for user listing. My backends.php for the 
server looks like this:

$servers['imap'] = array(
     // ENABLED by default; will connect to IMAP port on local server
     'disabled' => false,
     'name' => 'IMAP Server',
     'hostspec' => 'localhost',
     'hordeauth' => false,
     'protocol' => 'imap',
     'port' => 143,
     // Plaintext logins are disabled by default on IMAP servers (see RFC 3501
     // [6.2.3]), so TLS is the only guaranteed authentication available by
     // default.
     'secure' => 'tls',
  'spam' => array(
          'innocent' => array(
              'display' => true,

              // Email reporting driver
              'email' => null,
              'email_format' => 'digest',

              // Null reporting driver
              'null' => true,

              // Program reporting driver
              'program' => '/usr/bin/spamassassin -k'
          ),
          'spam' => array(
              'display' => false,

              // Email reporting driver
              'email' => null,
              'email_format' => 'digest',

              // Null reporting driver
              'null' => true,

              // Program reporting driver
              'program' => '/usr/bin/spamassassin -r'

          ),
         // // It is possible to directly define additional spam drivers.
         // // The 'drivers' array should contain a list of driver objects
         // // (these classes must implement the IMP_Spam_Base class).
         // 'drivers' => array(
         //     new IMP_Example_Spam_Driver()
         // )
     ),
);

That's not the answer to my question. Maybe I should have been more 
specific: does your backend *configuration* in IMP allow user listing?

I use Dovecot 1.3 with the IMAP protocol.

Cannot reproduce. Does your IMP backend allow user listing?

Since updating to Webmail 5.1.0 when I try to open the Caldav Account 
Link provided by kronolith like 
https://www.xxx.de/webmail/rpc/principals/username/ I get a html 
Credentials request. When entering the right credentials I get a
<d:error><s:exception>Sabre\DAV\Exception\NotFound</s:exception><s:message>User username does not 
exist</s:message><s:sabredav-version>1.8.4</s:sabredav-version></d:error>

althought the user "username" exists. I do the authentication via IMP. 
This error did not appear with the beta packages.