Looks like default values can't be null.

A null value here for some reason prevents the pref from being
set later on.

horde/config/prefs.php

@@ -758,7 +758,7 @@
 );
 // Handled by the special.
 $_prefs['activesync_identity'] = array(
-    'value' => null
+    'value' => 0
 );