diff --git a/src/AsyncEventSource.cpp b/src/AsyncEventSource.cpp
index f2914df54..c8bc6adf3 100644
--- a/src/AsyncEventSource.cpp
+++ b/src/AsyncEventSource.cpp
@@ -173,7 +173,9 @@ AsyncEventSourceClient::AsyncEventSourceClient(AsyncWebServerRequest *request, A
 }
 
 AsyncEventSourceClient::~AsyncEventSourceClient(){
-   _messageQueue.free();
+  _lockmq.lock();
+  _messageQueue.free();
+  _lockmq.unlock();
   close();
 }
 
@@ -184,33 +186,41 @@ void AsyncEventSourceClient::_queueMessage(AsyncEventSourceMessage *dataMessage)
     delete dataMessage;
     return;
   }
+  //length() is not thread-safe, thus acquiring the lock before this call..
+  _lockmq.lock();
   if(_messageQueue.length() >= SSE_MAX_QUEUED_MESSAGES){
-      ets_printf("ERROR: Too many messages queued\n");
-      delete dataMessage;
+    ets_printf("ERROR: Too many messages queued\n");
+    delete dataMessage;
   } else {
-      _messageQueue.add(dataMessage);
+    _messageQueue.add(dataMessage);
+    // runqueue trigger when new messages added
+    if(_client->canSend()) {
+      _runQueue();
+    }
   }
-  if(_client->canSend())
-    _runQueue();
+  _lockmq.unlock();
 }
 
 void AsyncEventSourceClient::_onAck(size_t len, uint32_t time){
+  // Same here, acquiring the lock early
+  _lockmq.lock();
   while(len && !_messageQueue.isEmpty()){
     len = _messageQueue.front()->ack(len, time);
     if(_messageQueue.front()->finished())
       _messageQueue.remove(_messageQueue.front());
   }
-
   _runQueue();
+  _lockmq.unlock();
 }
 
 void AsyncEventSourceClient::_onPoll(){
+  _lockmq.lock();
   if(!_messageQueue.isEmpty()){
     _runQueue();
   }
+  _lockmq.unlock();
 }
 
-
 void AsyncEventSourceClient::_onTimeout(uint32_t time __attribute__((unused))){
   _client->close(true);
 }
@@ -225,7 +235,7 @@ void AsyncEventSourceClient::close(){
     _client->close();
 }
 
-void AsyncEventSourceClient::write(const char * message, size_t len){
+void AsyncEventSourceClient::_write(const char * message, size_t len){
   _queueMessage(new AsyncEventSourceMessage(message, len));
 }
 
@@ -234,15 +244,23 @@ void AsyncEventSourceClient::send(const char *message, const char *event, uint32
   _queueMessage(new AsyncEventSourceMessage(ev.c_str(), ev.length()));
 }
 
-void AsyncEventSourceClient::_runQueue(){
-  while(!_messageQueue.isEmpty() && _messageQueue.front()->finished()){
-    _messageQueue.remove(_messageQueue.front());
-  }
+size_t AsyncEventSourceClient::packetsWaiting() const {
+    size_t len;
+    _lockmq.lock();
+    len = _messageQueue.length();
+    _lockmq.unlock();
+    return len;
+}
 
-  for(auto i = _messageQueue.begin(); i != _messageQueue.end(); ++i)
-  {
-    if(!(*i)->sent())
+void AsyncEventSourceClient::_runQueue() {
+  // Calls to this private method now already protected by _lockmq acquisition
+  // so no extra call of _lockmq.lock() here..
+  for (auto i = _messageQueue.begin(); i != _messageQueue.end(); ++i) {
+    // If it crashes here, iterator (i) has been invalidated as _messageQueue
+    // has been changed... (UL 2020-11-15: Not supposed to happen any more ;-) )
+    if (!(*i)->sent()) {
       (*i)->send(_client);
+    }
   }
 }
 
@@ -276,17 +294,22 @@ void AsyncEventSource::_addClient(AsyncEventSourceClient * client){
     client->write((const char *)temp, 2053);
     free(temp);
   }*/
-  
+  AsyncWebLockGuard l(_client_queue_lock);
   _clients.add(client);
   if(_connectcb)
     _connectcb(client);
 }
 
 void AsyncEventSource::_handleDisconnect(AsyncEventSourceClient * client){
+  AsyncWebLockGuard l(_client_queue_lock);
   _clients.remove(client);
 }
 
 void AsyncEventSource::close(){
+  // While the whole loop is not done, the linked list is locked and so the
+  // iterator should remain valid even when AsyncEventSource::_handleDisconnect()
+  // is called very early
+  AsyncWebLockGuard l(_client_queue_lock);
   for(const auto &c: _clients){
     if(c->connected())
       c->close();
@@ -295,37 +318,39 @@ void AsyncEventSource::close(){
 
 // pmb fix
 size_t AsyncEventSource::avgPacketsWaiting() const {
-  if(_clients.isEmpty())
+  size_t aql = 0;
+  uint32_t nConnectedClients = 0;
+  AsyncWebLockGuard l(_client_queue_lock);
+  if (_clients.isEmpty()) {
     return 0;
-  
-  size_t    aql=0;
-  uint32_t  nConnectedClients=0;
-  
+  }
   for(const auto &c: _clients){
     if(c->connected()) {
-      aql+=c->packetsWaiting();
+      aql += c->packetsWaiting();
       ++nConnectedClients;
     }
   }
-//  return aql / nConnectedClients;
-  return ((aql) + (nConnectedClients/2))/(nConnectedClients); // round up
+  return ((aql) + (nConnectedClients/2)) / (nConnectedClients); // round up
 }
 
-void AsyncEventSource::send(const char *message, const char *event, uint32_t id, uint32_t reconnect){
-
-
+void AsyncEventSource::send(
+    const char *message, const char *event, uint32_t id, uint32_t reconnect){
   String ev = generateEventMessage(message, event, id, reconnect);
+  AsyncWebLockGuard l(_client_queue_lock);
   for(const auto &c: _clients){
     if(c->connected()) {
-      c->write(ev.c_str(), ev.length());
+      c->_write(ev.c_str(), ev.length());
     }
   }
 }
 
 size_t AsyncEventSource::count() const {
-  return _clients.count_if([](AsyncEventSourceClient *c){
-    return c->connected();
-  });
+  size_t n_clients;
+  AsyncWebLockGuard l(_client_queue_lock);
+  n_clients = _clients.count_if([](AsyncEventSourceClient *c){
+                                    return c->connected();
+                                });
+  return n_clients;
 }
 
 bool AsyncEventSource::canHandle(AsyncWebServerRequest *request){
diff --git a/src/AsyncEventSource.h b/src/AsyncEventSource.h
index b097fa623..6c8fad7b0 100644
--- a/src/AsyncEventSource.h
+++ b/src/AsyncEventSource.h
@@ -72,6 +72,8 @@ class AsyncEventSourceClient {
     AsyncEventSource *_server;
     uint32_t _lastId;
     LinkedList<AsyncEventSourceMessage *> _messageQueue;
+    // ArFi 2020-08-27 for protecting/serializing _messageQueue
+    AsyncPlainLock _lockmq;
     void _queueMessage(AsyncEventSourceMessage *dataMessage);
     void _runQueue();
 
@@ -82,12 +84,12 @@ class AsyncEventSourceClient {
 
     AsyncClient* client(){ return _client; }
     void close();
-    void write(const char * message, size_t len);
     void send(const char *message, const char *event=NULL, uint32_t id=0, uint32_t reconnect=0);
     bool connected() const { return (_client != NULL) && _client->connected(); }
     uint32_t lastId() const { return _lastId; }
-    size_t  packetsWaiting() const { return _messageQueue.length(); }
+    size_t  packetsWaiting() const;
 
+    void _write(const char * message, size_t len);
     //system callbacks (do not call)
     void _onAck(size_t len, uint32_t time);
     void _onPoll(); 
@@ -99,7 +101,11 @@ class AsyncEventSource: public AsyncWebHandler {
   private:
     String _url;
     LinkedList<AsyncEventSourceClient *> _clients;
+    // Same as for individual messages, protect mutations of _clients list
+    // since simultaneous access from different tasks is possible
+    AsyncWebLock _client_queue_lock;
     ArEventHandlerFunction _connectcb;
+
   public:
     AsyncEventSource(const String& url);
     ~AsyncEventSource();
@@ -108,7 +114,7 @@ class AsyncEventSource: public AsyncWebHandler {
     void close();
     void onConnect(ArEventHandlerFunction cb);
     void send(const char *message, const char *event=NULL, uint32_t id=0, uint32_t reconnect=0);
-    size_t count() const; //number clinets connected
+    size_t count() const; //number clients connected
     size_t  avgPacketsWaiting() const;
 
     //system callbacks (do not call)
diff --git a/src/AsyncWebSynchronization.h b/src/AsyncWebSynchronization.h
index f36c52dcf..02ad2dc61 100644
--- a/src/AsyncWebSynchronization.h
+++ b/src/AsyncWebSynchronization.h
@@ -7,6 +7,38 @@
 
 #ifdef ESP32
 
+// This is the ESP32 version of the Sync Lock, using the FreeRTOS Semaphore
+// Modified 'AsyncWebLock' to just only use mutex since pxCurrentTCB is not
+// always available. According to example by Arjan Filius, changed name,
+// added unimplemented version for ESP8266
+class AsyncPlainLock
+{
+private:
+  SemaphoreHandle_t _lock;
+
+public:
+  AsyncPlainLock() {
+    _lock = xSemaphoreCreateBinary();
+    // In this fails, the system is likely that much out of memory that
+    // we should abort anyways. If assertions are disabled, nothing is lost..
+    assert(_lock);
+    xSemaphoreGive(_lock);
+  }
+
+  ~AsyncPlainLock() {
+    vSemaphoreDelete(_lock);
+  }
+
+  bool lock() const {
+      xSemaphoreTake(_lock, portMAX_DELAY);
+      return true;
+  }
+
+  void unlock() const {
+    xSemaphoreGive(_lock);
+  }
+};
+
 // This is the ESP32 version of the Sync Lock, using the FreeRTOS Semaphore
 class AsyncWebLock
 {
@@ -17,6 +49,9 @@ class AsyncWebLock
 public:
   AsyncWebLock() {
     _lock = xSemaphoreCreateBinary();
+    // In this fails, the system is likely that much out of memory that
+    // we should abort anyways. If assertions are disabled, nothing is lost..
+    assert(_lock);
     _lockedBy = NULL;
     xSemaphoreGive(_lock);
   }
@@ -61,6 +96,10 @@ class AsyncWebLock
   void unlock() const {
   }
 };
+
+// Same for AsyncPlainLock, for ESP8266 this is just the unimplemented version above. 
+using AsyncPlainLock = AsyncWebLock;
+
 #endif
 
 class AsyncWebLockGuard
@@ -84,4 +123,4 @@ class AsyncWebLockGuard
   }
 };
 
-#endif // ASYNCWEBSYNCHRONIZATION_H_
\ No newline at end of file
+#endif // ASYNCWEBSYNCHRONIZATION_H_