Index: binaries/data/mods/public/gui/gamesetup/Controls/StartGameControl.js
===================================================================
--- binaries/data/mods/public/gui/gamesetup/Controls/StartGameControl.js
+++ binaries/data/mods/public/gui/gamesetup/Controls/StartGameControl.js
@@ -20,6 +20,7 @@
 		this.gameStarted = false;
 
 		netMessages.registerNetMessageHandler("start", this.switchToLoadingPage.bind(this));
+		netMessages.registerNetMessageHandler("startfailed", this.switchBack.bind(this));
 	}
 
 	registerLaunchGameHandler(handler)
@@ -37,17 +38,23 @@
 		if (g_IsNetworked)
 			Engine.StartNetworkGame();
 		else
-		{
 			Engine.StartGame(g_GameAttributes, g_PlayerAssignments.local.player);
-			this.switchToLoadingPage();
-		}
+		this.switchToLoadingPage();
 	}
 
 	switchToLoadingPage()
 	{
-		Engine.SwitchGuiPage("page_loading.xml", {
+		Engine.PushGuiPage("page_loading.xml", {
 			"attribs": g_GameAttributes,
 			"playerAssignments": g_PlayerAssignments
 		});
 	}
+
+	switchBack(msg)
+	{
+		error(sprintf(translate("Game start failed: %s"),
+			msg.reason || translate("Unknown reason")));
+		Engine.PopGuiPage();
+		Engine.ResetCursor();
+	}
 }
Index: binaries/data/mods/public/gui/gamesetup/NetMessages/NetMessages.js
===================================================================
--- binaries/data/mods/public/gui/gamesetup/NetMessages/NetMessages.js
+++ binaries/data/mods/public/gui/gamesetup/NetMessages/NetMessages.js
@@ -67,5 +67,6 @@
 	"netstatus",
 	"netwarn",
 	"players",
-	"start"
+	"start",
+	"startfailed"
 ];
Index: binaries/data/mods/public/gui/gamesetup/Pages/GameSetupPage/Panels/Buttons/StartGameButton.js
===================================================================
--- binaries/data/mods/public/gui/gamesetup/Pages/GameSetupPage/Panels/Buttons/StartGameButton.js
+++ binaries/data/mods/public/gui/gamesetup/Pages/GameSetupPage/Panels/Buttons/StartGameButton.js
@@ -30,7 +30,7 @@
 	update()
 	{
 		let isEveryoneReady = this.isEveryoneReady();
-		this.startGameButton.enabled = !this.gameStarted && isEveryoneReady;
+		this.startGameButton.enabled = true;//!this.gameStarted && isEveryoneReady;
 		this.startGameButton.tooltip =
 			!g_IsNetworked || isEveryoneReady ?
 				this.ReadyTooltip :
Index: source/network/NetClient.h
===================================================================
--- source/network/NetClient.h
+++ source/network/NetClient.h
@@ -249,6 +249,7 @@
 	static bool OnPlayerAssignment(void* context, CFsmEvent* event);
 	static bool OnInGame(void* context, CFsmEvent* event);
 	static bool OnGameStart(void* context, CFsmEvent* event);
+	static bool OnGameStartFailed(void* context, CFsmEvent* event);
 	static bool OnJoinSyncStart(void* context, CFsmEvent* event);
 	static bool OnJoinSyncEndCommandBatch(void* context, CFsmEvent* event);
 	static bool OnRejoined(void* context, CFsmEvent* event);
Index: source/network/NetClient.cpp
===================================================================
--- source/network/NetClient.cpp
+++ source/network/NetClient.cpp
@@ -102,6 +102,7 @@
 	AddTransition(NCS_PREGAME, (uint)NMT_CLIENT_TIMEOUT, NCS_PREGAME, (void*)&OnClientTimeout, context);
 	AddTransition(NCS_PREGAME, (uint)NMT_CLIENT_PERFORMANCE, NCS_PREGAME, (void*)&OnClientPerformance, context);
 	AddTransition(NCS_PREGAME, (uint)NMT_GAME_START, NCS_LOADING, (void*)&OnGameStart, context);
+	AddTransition(NCS_PREGAME, (uint)NMT_GAME_START_FAILED, NCS_PREGAME, (void*)&OnGameStartFailed, context);
 	AddTransition(NCS_PREGAME, (uint)NMT_JOIN_SYNC_START, NCS_JOIN_SYNCING, (void*)&OnJoinSyncStart, context);
 
 	AddTransition(NCS_JOIN_SYNCING, (uint)NMT_CHAT, NCS_JOIN_SYNCING, (void*)&OnChat, context);
@@ -663,6 +664,21 @@
 	return true;
 }
 
+bool CNetClient::OnGameStartFailed(void* context, CFsmEvent* event)
+{
+	ENSURE(event->GetType() == (uint)NMT_GAME_START_FAILED);
+
+	CNetClient* client = static_cast<CNetClient*>(context);
+	CGameStartFailedMessage* message = static_cast<CGameStartFailedMessage*>(event->GetParamRef());
+
+	client->PushGuiMessage(
+		"type", "startfailed",
+		"reason", message->m_Reason
+	);
+
+	return true;
+}
+
 bool CNetClient::OnJoinSyncStart(void* context, CFsmEvent* event)
 {
 	ENSURE(event->GetType() == (uint)NMT_JOIN_SYNC_START);
Index: source/network/NetMessage.cpp
===================================================================
--- source/network/NetMessage.cpp
+++ source/network/NetMessage.cpp
@@ -183,6 +183,10 @@
 		pNewMessage = new CGameStartMessage;
 		break;
 
+	case NMT_GAME_START_FAILED:
+		pNewMessage = new CGameStartFailedMessage;
+		break;
+
 	case NMT_END_COMMAND_BATCH:
 		pNewMessage = new CEndCommandBatchMessage;
 		break;
Index: source/network/NetMessages.h
===================================================================
--- source/network/NetMessages.h
+++ source/network/NetMessages.h
@@ -28,13 +28,14 @@
 
 #define PS_PROTOCOL_MAGIC                         0x5073013f	// 'P', 's', 0x01, '?'
 #define PS_PROTOCOL_MAGIC_RESPONSE                0x50630121	// 'P', 'c', 0x01, '!'
-#define PS_PROTOCOL_VERSION                       0x01010015	// Arbitrary protocol
+#define PS_PROTOCOL_VERSION                       0x01010016	// Arbitrary protocol
 #define PS_DEFAULT_PORT                           0x5073		// 'P', 's'
 
 // Set when lobby authentication is required. Used in the SrvHandshakeResponseMessage.
 #define PS_NETWORK_FLAG_REQUIRE_LOBBYAUTH         0x1
 
-// Defines the list of message types. The order of the list must not change.
+// Defines the list of message types.
+// The order of the list must not change (unless the protocol version is changed).
 // The message types having a negative value are used internally and not sent
 // over the network. The message types used for network communication have
 // positive values.
@@ -75,6 +76,7 @@
 
 	NMT_LOADED_GAME,
 	NMT_GAME_START,
+	NMT_GAME_START_FAILED,
 	NMT_END_COMMAND_BATCH,
 
 	NMT_SYNC_CHECK,	// OOS-detection hash checking
@@ -215,6 +217,10 @@
 START_NMT_CLASS_(GameStart, NMT_GAME_START)
 END_NMT_CLASS()
 
+START_NMT_CLASS_(GameStartFailed, NMT_GAME_START_FAILED)
+	NMT_FIELD(CStr, m_Reason)
+END_NMT_CLASS()
+
 START_NMT_CLASS_(EndCommandBatch, NMT_END_COMMAND_BATCH)
 	NMT_FIELD_INT(m_Turn, u32, 4)
 	NMT_FIELD_INT(m_TurnLength, u32, 2)
Index: source/network/NetServer.cpp
===================================================================
--- source/network/NetServer.cpp
+++ source/network/NetServer.cpp
@@ -1485,7 +1485,10 @@
 	for (std::pair<const CStr, PlayerAssignment>& player : m_PlayerAssignments)
 		if (player.second.m_Enabled && player.second.m_PlayerID != -1 && player.second.m_Status == 0)
 		{
-			LOGERROR("Tried to start the game without player \"%s\" being ready!", utf8_from_wstring(player.second.m_Name).c_str());
+			CGameStartFailedMessage gameStartFailed;
+			gameStartFailed.m_Reason = "Player " + player.second.m_Name.ToUTF8() + " is not ready";
+			Broadcast(&gameStartFailed, { NSS_PREGAME });
+
 			return;
 		}