diff --git a/lib/ext/Syncroton/Command/Sync.php b/lib/ext/Syncroton/Command/Sync.php
--- a/lib/ext/Syncroton/Command/Sync.php
+++ b/lib/ext/Syncroton/Command/Sync.php
@@ -844,6 +844,8 @@
                             $this->_logger->warn(__METHOD__ . '::' . __LINE__ . " unable to convert entry to xml: " . $e->getMessage());
                         if ($this->_logger instanceof Zend_Log)
                             $this->_logger->debug(__METHOD__ . '::' . __LINE__ . " unable to convert entry to xml: " . $e->getTraceAsString());
+                        // We bump collectionChanges anyways to make sure the windowSize still applies.
+                        $collectionChanges++;
                     }
                     
                     // mark as sent to the client, even the conversion to xml might have failed 
@@ -890,6 +892,8 @@
                     } catch (Exception $e) {
                         if ($this->_logger instanceof Zend_Log) 
                             $this->_logger->warn(__METHOD__ . '::' . __LINE__ . " unable to convert entry to xml: " . $e->getMessage());
+                        // We bump collectionChanges anyways to make sure the windowSize still applies.
+                        $collectionChanges++;
                     }
 
                     unset($serverModifications['changed'][$id]);    
@@ -915,6 +919,8 @@
                     } catch (Exception $e) {
                         if ($this->_logger instanceof Zend_Log) 
                             $this->_logger->warn(__METHOD__ . '::' . __LINE__ . " unable to convert entry to xml: " . $e->getMessage());
+                        // We bump collectionChanges anyways to make sure the windowSize still applies.
+                        $collectionChanges++;
                     }
                     
                     unset($serverModifications['deleted'][$id]);