diff --git a/lib/remote/apilistener-filesync.cpp b/lib/remote/apilistener-filesync.cpp
index 87aae6104..411028630 100644
--- a/lib/remote/apilistener-filesync.cpp
+++ b/lib/remote/apilistener-filesync.cpp
@@ -15,6 +15,8 @@ using namespace icinga;
 
 REGISTER_APIFUNCTION(Update, config, &ApiListener::ConfigUpdateHandler);
 
+boost::mutex ApiListener::m_ConfigSyncStageLock;
+
 void ApiListener::ConfigGlobHandler(ConfigDirInformation& config, const String& path, const String& file)
 {
 	CONTEXT("Creating config update for file '" + file + "'");
@@ -274,6 +276,11 @@ Value ApiListener::ConfigUpdateHandler(const MessageOrigin::Ptr& origin, const D
 		return Empty;
 	}
 
+	/* Only one transaction is allowed, concurrent message handlers need to wait.
+	 * This affects two parent endpoints sending the config in the same moment.
+	 */
+	boost::mutex::scoped_lock lock(m_ConfigSyncStageLock);
+
 	Log(LogInformation, "ApiListener")
 		<< "Applying config update from endpoint '" << origin->FromClient->GetEndpoint()->GetName()
 		<< "' of zone '" << GetFromZoneName(origin->FromZone) << "'.";
diff --git a/lib/remote/apilistener.hpp b/lib/remote/apilistener.hpp
index 5818ec994..022ad8281 100644
--- a/lib/remote/apilistener.hpp
+++ b/lib/remote/apilistener.hpp
@@ -165,6 +165,8 @@ private:
 	void RemoveStatusFile();
 
 	/* filesync */
+	static boost::mutex m_ConfigSyncStageLock;
+
 	static ConfigDirInformation LoadConfigDir(const String& dir);
 	static Dictionary::Ptr MergeConfigUpdate(const ConfigDirInformation& config);
 	static bool UpdateConfigDir(const ConfigDirInformation& oldConfig, const ConfigDirInformation& newConfig, const String& configDir, bool authoritative);