diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/SEBClientEventBatchService.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/SEBClientEventBatchService.java
index 1a29bb0b..7a262eca 100644
--- a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/SEBClientEventBatchService.java
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/SEBClientEventBatchService.java
@@ -160,10 +160,16 @@ public class SEBClientEventBatchService {
                 return eventData;
             }
 
-            System.out.println("******* storeNotifications: " + eventData);
+            ClientConnectionDataInternal clientConnection = null;
+            synchronized (ExamSessionCacheService.CLIENT_CONNECTION_CREATION_LOCK) {
+                clientConnection = this.examSessionCacheService
+                        .getClientConnection(eventData.connectionToken);
+            }
 
-            final ClientConnectionDataInternal clientConnection = this.examSessionCacheService
-                    .getClientConnection(eventData.connectionToken);
+            if (clientConnection == null) {
+                log.error("Failed to get ClientConnectionDataInternal for: {}", eventData.connectionToken);
+                return null;
+            }
 
             final Pair<NotificationType, String> typeAndPlainText =
                     ClientNotification.extractTypeAndPlainText(eventData.event.text);
@@ -200,8 +206,12 @@ public class SEBClientEventBatchService {
 
     private ClientEventRecord toEventRecord(final EventData eventData) {
         try {
-            final ClientConnectionDataInternal clientConnection = this.examSessionCacheService
-                    .getClientConnection(eventData.connectionToken);
+
+            ClientConnectionDataInternal clientConnection = null;
+            synchronized (ExamSessionCacheService.CLIENT_CONNECTION_CREATION_LOCK) {
+                clientConnection = this.examSessionCacheService
+                        .getClientConnection(eventData.connectionToken);
+            }
 
             if (clientConnection == null) {
                 log.warn("Failed to retrieve ClientConnection for token {}. Skip this event",
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/SEBClientPingBatchService.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/SEBClientPingBatchService.java
index 815aad01..44a86a95 100644
--- a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/SEBClientPingBatchService.java
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/SEBClientPingBatchService.java
@@ -91,11 +91,16 @@ public class SEBClientPingBatchService {
             return;
         }
 
-        final ClientConnectionDataInternal activeClientConnection = this.examSessionCacheService
-                .getClientConnection(connectionToken);
+        ClientConnectionDataInternal activeClientConnection = null;
+        synchronized (ExamSessionCacheService.CLIENT_CONNECTION_CREATION_LOCK) {
+            activeClientConnection = this.examSessionCacheService
+                    .getClientConnection(connectionToken);
+        }
 
         if (activeClientConnection != null) {
             activeClientConnection.notifyPing(timestamp);
+        } else {
+            log.error("Failed to get ClientConnectionDataInternal for: {}", connectionToken);
         }
 
         if (instructionConfirm != StringUtils.EMPTY) {