diff --git a/doc/requirements.txt b/doc/requirements.txt
index dc1d7efca4..b2f7d0e0c4 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -9,4 +9,8 @@ exhale
 Jinja2
 sphinxcontrib-youtube<=1.2
 sphinxcontrib-apidoc
-
+sphinxcontrib-applehelp<=1.0.7        # 2024-01-15: Newer packages needs Sphinx 5 or up, but our old sphinx-book-theme 0.3.3 requires Sphinx 4
+sphinxcontrib-devhelp<=1.0.5          # 2024-01-15: Newer packages need Sphinx 5 or up, but our old sphinx-book-theme 0.3.3 requires Sphinx 4
+sphinxcontrib-htmlhelp<=2.0.4         # 2024-01-15: Newer packages (probably) need Sphinx 5 or up, but our old sphinx-book-theme 0.3.3 requires Sphinx 4
+sphinxcontrib-qthelp<=1.0.6           # 2024-01-15: Newer packages (probably) need Sphinx 5 or up, but our old sphinx-book-theme 0.3.3 requires Sphinx 4
+sphinxcontrib-serializinghtml<=1.1.9  # 2024-01-15: Newer packages (probably) need Sphinx 5 or up, but our old sphinx-book-theme 0.3.3 requires Sphinx 4
\ No newline at end of file
diff --git a/ecal/core/src/io/udp_receiver.cpp b/ecal/core/src/io/udp_receiver.cpp
index d7e6925fb6..012f596006 100644
--- a/ecal/core/src/io/udp_receiver.cpp
+++ b/ecal/core/src/io/udp_receiver.cpp
@@ -73,25 +73,34 @@ namespace eCAL
   bool CUDPReceiver::Destroy()
   {
     if (!m_socket_impl) return(false);
+
+    const std::lock_guard<std::mutex> lock(m_socket_mtx);
     m_socket_impl.reset();
+
     return(true);
   }
 
   bool CUDPReceiver::AddMultiCastGroup(const char* ipaddr_)
   {
     if (!m_socket_impl) return(false);
+    
+    const std::lock_guard<std::mutex> lock(m_socket_mtx);
     return(m_socket_impl->AddMultiCastGroup(ipaddr_));
   }
 
   bool CUDPReceiver::RemMultiCastGroup(const char* ipaddr_)
   {
     if (!m_socket_impl) return(false);
+
+    const std::lock_guard<std::mutex> lock(m_socket_mtx);
     return(m_socket_impl->RemMultiCastGroup(ipaddr_));
   }
 
   size_t CUDPReceiver::Receive(char* buf_, size_t len_, int timeout_, ::sockaddr_in* address_ /* = nullptr */)
   {
     if (!m_socket_impl) return(0);
+
+    const std::lock_guard<std::mutex> lock(m_socket_mtx);
     return(m_socket_impl->Receive(buf_, len_, timeout_, address_));
   }
 }
diff --git a/ecal/core/src/io/udp_receiver.h b/ecal/core/src/io/udp_receiver.h
index 500aaeec6c..6a976bde6f 100644
--- a/ecal/core/src/io/udp_receiver.h
+++ b/ecal/core/src/io/udp_receiver.h
@@ -24,6 +24,7 @@
 #pragma once
 
 #include <memory>
+#include <mutex>
 #include "ecal_receiver.h"
 
 namespace eCAL
@@ -44,7 +45,8 @@ namespace eCAL
     size_t Receive(char* buf_, size_t len_, int timeout_, ::sockaddr_in* address_ = nullptr) override;
 
   protected:
-    bool m_use_npcap;
+    bool                              m_use_npcap;
+    std::mutex                        m_socket_mtx;
     std::shared_ptr<CUDPReceiverBase> m_socket_impl;
   };
 }