diff --git a/os/windows/installer/install.nsi b/os/windows/installer/install.nsi
index d4448dcf9b..d437bb820a 100644
--- a/os/windows/installer/install.nsi
+++ b/os/windows/installer/install.nsi
@@ -102,6 +102,9 @@ Page custom SelectCDEnter SelectCDExit ": TTD folder"
 ;--------------------------------------------------------------
 ; (Core) OpenTTD install section. Copies all internal game data
 Section "!OpenTTD" Section1
+	; Make sure to be upgraded OpenTTD is not running
+	Call CheckOpenTTDRunning
+
 	; Overwrite files by default, but don't complain on failure
 	SetOverwrite try
 
@@ -520,7 +523,6 @@ Abort:
 Done:
 FunctionEnd
 
-
 ;-------------------------------------------------------------------------------
 ; Check whether we're not running an installer for NT on 9x and vice versa
 Function CheckWindowsVersion
@@ -540,6 +542,23 @@ Abort:
 Done:
 FunctionEnd
 
+;-------------------------------------------------------------------------------
+; Check whether OpenTTD is running
+Function CheckOpenTTDRunning
+	IfFileExists "$INSTDIR\openttd.exe" 0 Done
+Retry:
+	FindProcDLL::FindProc "openttd.exe"
+	Pop $R0
+	IntCmp $R0 1 0 Done
+	ClearErrors
+	Delete "$INSTDIR\openttd.exe"
+	IfErrors 0 Done
+	ClearErrors
+	MessageBox MB_RETRYCANCEL|MB_ICONEXCLAMATION "OpenTTD is running. Please close it and retry." IDRETRY Retry
+	Abort
+Done:
+FunctionEnd
+
 Var OLDVERSION
 Var UninstallString
 
diff --git a/src/network/core/tcp.cpp b/src/network/core/tcp.cpp
index 39f90569f2..6f04b12c2b 100644
--- a/src/network/core/tcp.cpp
+++ b/src/network/core/tcp.cpp
@@ -82,6 +82,8 @@ void NetworkTCPSocketHandler::Send_Packet(Packet *packet)
  *      data right now (full network-buffer, it happens ;))
  *   3) sending took too long
  * @param closing_down Whether we are closing down the connection.
+ * @return \c true if a (part of a) packet could be sent and
+ *         the connection is not closed yet.
  */
 bool NetworkTCPSocketHandler::Send_Packets(bool closing_down)
 {
diff --git a/src/network/network.cpp b/src/network/network.cpp
index c833090bf8..c7f72a47fb 100644
--- a/src/network/network.cpp
+++ b/src/network/network.cpp
@@ -681,7 +681,7 @@ static void NetworkClose()
 	FOR_ALL_CLIENT_SOCKETS(cs) {
 		if (!_network_server) {
 			SEND_COMMAND(PACKET_CLIENT_QUIT)();
-			cs->Send_Packets();
+			cs->Send_Packets(true);
 		}
 		NetworkCloseClient(cs, NETWORK_RECV_STATUS_CONN_LOST);
 	}
@@ -1012,9 +1012,7 @@ static void NetworkSend()
 	NetworkClientSocket *cs;
 	FOR_ALL_CLIENT_SOCKETS(cs) {
 		if (cs->writable) {
-			cs->Send_Packets();
-
-			if (cs->status == STATUS_MAP) {
+			if (cs->Send_Packets() && cs->status == STATUS_MAP) {
 				/* This client is in the middle of a map-send, call the function for that */
 				SEND_COMMAND(PACKET_SERVER_MAP)(cs);
 			}
diff --git a/src/network/network_server.cpp b/src/network/network_server.cpp
index e7f3a64091..7761143c60 100644
--- a/src/network/network_server.cpp
+++ b/src/network/network_server.cpp
@@ -412,8 +412,7 @@ DEF_SERVER_SEND_COMMAND(PACKET_SERVER_MAP)
 		}
 
 		/* Send all packets (forced) and check if we have send it all */
-		cs->Send_Packets();
-		if (cs->IsPacketQueueEmpty()) {
+		if (cs->Send_Packets() && cs->IsPacketQueueEmpty()) {
 			/* All are sent, increase the sent_packets */
 			sent_packets *= 2;
 		} else {