diff --git a/lib/kolab_sync_data.php b/lib/kolab_sync_data.php
--- a/lib/kolab_sync_data.php
+++ b/lib/kolab_sync_data.php
@@ -484,7 +484,7 @@
             throw new Syncroton_Exception_Status_Sync(Syncroton_Exception_Status_Sync::SYNC_SERVER_ERROR);
         }
 
-        return $entry['uid'];
+        return $entry['_serverId'];
     }
 
     /**
@@ -505,13 +505,13 @@
         }
 
         $entry = $this->toKolab($entry, $folderId, $oldEntry);
-        $entry = $this->updateObject($folderId, $serverId,  $entry);
+        $entry = $this->updateObject($folderId, $serverId, $entry);
 
         if (empty($entry)) {
             throw new Syncroton_Exception_Status_Sync(Syncroton_Exception_Status_Sync::SYNC_SERVER_ERROR);
         }
 
-        return $entry['uid'];
+        return $entry['_serverId'];
     }
 
     /**
@@ -602,8 +602,8 @@
                 if (!is_array($uids)) {
                     $error = true;
                 }
-                else {
-                    $result = array_merge($result, $uids);
+                else if (!empty($uids)) {
+                    $result = array_merge($result, $this->applyServerId($uids, $folder));
                 }
                 break;
             }
@@ -641,8 +641,8 @@
 
                 case self::RESULT_UID:
                     $uids = $folder->get_uids($tag_filter);
-                    if (is_array($uids)) {
-                        $result = array_unique(array_merge($result, $uids));
+                    if (is_array($uids) && !empty($uids)) {
+                        $result = array_unique(array_merge($result, $this->applyServerId($uids, $folder)));
                     }
 
                     break;
@@ -944,10 +944,37 @@
             $foldername = $this->backend->folder_id2name($folderid, $this->device->deviceid);
             $folder     = $this->getFolderObject($foldername);
 
-            if ($folder && $folder->valid && ($object = $folder->get_object($entryid))) {
-                $object['_folderid'] = $folderid;
+            if ($folder && $folder->valid) {
+                $crc = null;
+                $uid = $entryid;
+
+                // See self::serverId() for full explanation
+                // Use (slower) UID prefix matching...
+                if (preg_match('/^CRC([0-9A-Fa-f]{8})(.+)$/', $uid, $matches)) {
+                    $crc = $matches[1];
+                    $uid = $matches[2];
+
+                    if (strlen($entryid) >= 64) {
+                        foreach ($folder->select(array(array('uid', '~*', $uid))) as $object) {
+                            if (($object['uid'] == $uid || strpos($object['uid'], $uid) === 0)
+                                && $crc == $this->objectCRC($object['uid'], $folder)
+                            ) {
+                                $object['_folderid'] = $folderid;
+                                return $object;
+                            }
+                        }
+
+                        continue;
+                    }
+                }
 
-                return $object;
+                // Or (faster) strict UID matching...
+                if (($object = $folder->get_object($uid))
+                    && ($crc === null || $crc == $this->objectCRC($object['uid'], $folder))
+                ) {
+                    $object['_folderid'] = $folderid;
+                    return $object;
+                }
             }
         }
     }
@@ -981,6 +1008,8 @@
                 $this->setKolabTags($data['uid'], $tags);
             }
 
+            $data['_serverId'] = $this->serverId($data['uid'], $folder);
+
             return $data;
         }
     }
@@ -1006,6 +1035,8 @@
                     $this->setKolabTags($data['uid'], $tags);
                 }
 
+                $data['_serverId'] = $this->serverId($object['uid'], $folder);
+
                 return $data;
             }
         }
@@ -1021,7 +1052,7 @@
         if ($object) {
             $folder = $this->getFolderObject($object['_mailbox']);
 
-            if ($folder && $folder->valid && $folder->delete($entryid)) {
+            if ($folder && $folder->valid && $folder->delete($object['uid'])) {
                 if ($this->tag_categories) {
                     $this->setKolabTags($object['uid'], null);
                 }
@@ -1848,4 +1879,65 @@
 
         return false;
     }
+
+    /**
+     * Generate CRC-based ServerId from object UID
+     */
+    protected function serverId($uid, $folder)
+    {
+        if ($this->modelName == 'mail') {
+            return $uid;
+        }
+
+        // When ActiveSync communicates with the client, it refers to objects with a ServerId
+        // We can't use object UID for ServerId because:
+        //     - ServerId is limited to 64 chars,
+        //     - there can be multiple calendars with a copy of the same event.
+        //
+        // The solution is to; Take the original UID, and regardless of its length, execute the following:
+        //     - Hash the UID concatenated with the Folder ID using CRC32b,
+        //     - Prefix the UID with 'CRC' and the hash string,
+        //     - Tryncate the result to 64 characters.
+        //
+        // Searching for the server-side copy of the object now follows the logic;
+        //     - If the ServerId is prefixed with 'CRC', strip off the first 11 characters
+        //       and we search for the UID using the remainder;
+        //     - if the UID is shorter than 53 characters, it'll be the complete UID,
+        //     - if the UID is longer than 53 characters, it'll be the truncated UID,
+        //       and we search for a wildcard match of <uid>*
+        // When multiple copies of the same event are found, the same CRC32b hash can be used
+        // on the events metadata (i.e. the copy's UID and Folder ID), and compared with the CRC from the ServerId.
+
+        // ServerId is max. 64 characters, below we generate a string of max. 64 chars
+        // Note: crc32b is always 8 characters
+        return 'CRC' . $this->objectCRC($uid, $folder) . substr($uid, 0, 53);
+    }
+
+    /**
+     * Calculate checksum on object UID and folder UID
+     */
+    protected function objectCRC($uid, $folder)
+    {
+        if (!is_object($folder)) {
+            $folder = $this->getFolderObject($folder);
+        }
+
+        $folder_uid = $folder->get_uid();
+
+        return strtoupper(hash('crc32b', $folder_uid . $uid)); // always 8 chars
+    }
+
+    /**
+     * Apply serverId() on a set of uids
+     */
+    protected function applyServerId($uids, $folder)
+    {
+        if (!empty($uids) && $this->modelName != 'mail') {
+            $self = $this;
+            $func = function($uid) use ($self, $folder) { return $self->serverId($uid, $folder); };
+            $uids = array_map($func, $uids);
+        }
+
+        return $uids;
+    }
 }
diff --git a/lib/kolab_sync_data_calendar.php b/lib/kolab_sync_data_calendar.php
--- a/lib/kolab_sync_data_calendar.php
+++ b/lib/kolab_sync_data_calendar.php
@@ -413,14 +413,6 @@
                     continue 2;
                 }
                 break;
-
-            case 'uid':
-                // If UID is too long, use auto-generated UID (#1034)
-                // It's because UID is used as ServerId which cannot be longer than 64 chars
-                if (strlen($value) > 64) {
-                    $value = null;
-                }
-                break;
             }
 
             $this->setKolabDataItem($event, $name, $value);