Changes have been made in Git (master):

commit 2b590ff47770c8b204f7e2602e209f557fc4dc02
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed Mar 6 12:39:03 2013 -0500

     Prevent chicken/egg with the logger.

     If we attempt to load the logger before horde's configuration is
     fully loaded, we might not have the the logger settings available.
     This will cause the injector factory to return a null logger. Since
     the injector acts as a singleton in this case, we may never obtain
     a correct logger.

     Fixes Bug: 12094

  framework/Core/lib/Horde.php |    4 +++-
  1 files changed, 3 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/2b590ff47770c8b204f7e2602e209f557fc4dc02

Actually, it looks like this is true for any rpc access. For some 
reason, the very first time we get the logger from the injector there 
is no logging config set. Since the injector acts as a singleton, we 
never get the proper logger.

Still tracing why this happens.

well i'm unsure about the technical reasons, but for a fact on our 
setup nothing is logged.

if i choose custom log, then everything logged via this->logger is 
written to the custom log, everything else gets lost.
if i choose horde log, then everything is lost.
this only happens for active sync sessions.

so please reopen this bug. tell me if you need more info

$this->_logger is only different than the logger returned from the 
injector if you select custom AS logging. Even then, only the 
ActiveSync library uses $this->_logger. The global logger available 
from the injector (which is what Horde::logMessage() uses) is unchanged.

When rpc.php runs as ActiveSync server Horde::LogMessage does not 
produce any output to the horde log, but only the custom 
$this->_logger does.

This is problematic since it hides relevant errors in other horde 
components, e.g. the registry for auth errors.