This isn't always an error.

We can have no collections sent from the client in a PARTIAL request
if the client is modifying the heartbeat/waitinterval/windowsize values.

framework/ActiveSync/lib/Horde/ActiveSync/Collections.php

@@ -732,8 +732,10 @@ public function initFullSync()
     public function initPartialSync()
     {
         if (empty($this->_collections)) {
-            $this->_logger->err('No collections in collection handler, no PARTIAL allowed.');
-            return false;
+            // PARTIAL is allowed without a <collection> tag if the
+            // waitinterval, heartbeat, or windowsize changed.
+            $this->_logger->err('No collections in collection handler, loading full collection set from cache.');
+            $this->_collections->loadCollectionsFromCache();
         }
         $this->_tempSyncCache = clone $this->_cache;
         $c = $this->_tempSyncCache->getCollections();