Index: binaries/data/mods/public/gui/session/messages.js
===================================================================
--- binaries/data/mods/public/gui/session/messages.js
+++ binaries/data/mods/public/gui/session/messages.js
@@ -39,6 +39,9 @@
 	"netwarn": msg => {
 		addNetworkWarning(msg);
 	},
+	"out-of-sync": msg => {
+		onNetworkOutOfSync(msg);
+	},
 	"players": msg => {
 		handlePlayerAssignmentsMessage(msg);
 	},
@@ -601,6 +604,58 @@
 	}
 }
 
+function onNetworkOutOfSync(msg)
+{
+	let txt = [
+		sprintf(translate("Out-of-sync error on turn %(turn)s."), {
+			"turn": msg.turn
+		}),
+
+		sprintf(translateWithContext("Out-Of-Sync", "Players: %(players)s"), {
+			"players": msg.players.join(translate(", "))
+		}),
+
+		msg.hash == msg.expectedHash ?
+			translateWithContext("Out-Of-Sync", "Your game state is identical to the hosts game state.") :
+			translateWithContext("Out-Of-Sync", "Your game state differs from the hosts game state."),
+
+		""
+	];
+
+	if (msg.turn > 1 && g_GameAttributes.settings.PlayerData.some(pData => pData && pData.AI))
+		txt.push(translateWithContext("Out-Of-Sync", "Rejoining Multiplayer games with AIs is not supported yet!"));
+	else
+	{
+		txt.push(
+			translateWithContext("Out-Of-Sync", "Ensure all players use the same mods."),
+			translateWithContext("Out-Of-Sync", 'Click on "Report Bugs" in the main menu to help fix this.'),
+			sprintf(translateWithContext("Out-Of-Sync", "Replay saved to %(filepath)s"), {
+				"filepath": escapeText(msg.path_replay)
+			}),
+			sprintf(translateWithContext("Out-Of-Sync", "Dumping current state to %(filepath)s"), {
+				"filepath": escapeText(msg.path_oos_dump)
+			})
+		);
+	}
+
+	messageBox(
+		600, 280,
+		txt.join("\n"),
+		translate("Out Of Sync")
+	);
+}
+
+function onReplayOutOfSync()
+{
+	messageBox(
+		500, 140,
+		translate("Out of synchronization error!") + "\n" +
+			// Translation: This is shown if replay is out of sync
+			translateWithContext("Out-Of-Sync", "The current game state is different from the original game state."),
+		translate("Out Of Sync")
+	);
+}
+
 function handlePlayerAssignmentsMessage(message)
 {
 	for (let guid in g_PlayerAssignments)
Index: binaries/data/mods/public/gui/session/session.xml
===================================================================
--- binaries/data/mods/public/gui/session/session.xml
+++ binaries/data/mods/public/gui/session/session.xml
@@ -30,6 +30,10 @@
 		onReplayFinished();
 	</action>
 
+	<action on="ReplayOutOfSync">
+		onReplayOutOfSync();
+	</action>
+
 	<action on="Press">
 		this.hidden = !this.hidden;
 	</action>
Index: source/network/NetClientTurnManager.cpp
===================================================================
--- source/network/NetClientTurnManager.cpp
+++ source/network/NetClientTurnManager.cpp
@@ -122,18 +122,29 @@
 
 	hash = Hexify(hash);
 
-	std::stringstream msg;
-	msg << "Out of sync on turn " << turn;
-
+	std::stringstream playerNamesString;
+	std::vector<CStr> playerNamesStrings;
+	playerNamesStrings.reserve(playerNames.size());
 	for (size_t i = 0; i < playerNames.size(); ++i)
-		msg << (i == 0 ? "\nPlayers: " : ", ") << utf8_from_wstring(playerNames[i].m_Name);
-
-	msg << "\n\n" << "Your game state is " << (expectedHash == hash ? "identical to" : "different from") << " the hosts game state.";
-
-	msg << "\n\n" << "Dumping current state to " << CStr(path.string8()).EscapeToPrintableASCII();
+	{
+		CStr name = utf8_from_wstring(playerNames[i].m_Name);
+		playerNamesString << (i == 0 ? "" : ", ") << name;
+		playerNamesStrings.push_back(name);
+	}
 
-	LOGERROR("%s", msg.str());
+	LOGERROR("Out-Of-Sync on turn %d\nPlayers: %s\nDumping state to %s", turn, playerNamesString.str().c_str(), path.string8());
 
-	if (g_GUI)
-		g_GUI->DisplayMessageBox(600, 350, L"Sync error", wstring_from_utf8(msg.str()));
+	ScriptInterface& scriptInterface = m_NetClient.GetScriptInterface();
+	JSContext* cx = scriptInterface.GetContext();
+	JSAutoRequest rq(cx);
+
+	JS::RootedValue msg(cx);
+	scriptInterface.Eval("({ 'type':'out-of-sync' })", &msg);
+	scriptInterface.SetProperty(msg, "turn", turn);
+	scriptInterface.SetProperty(msg, "players", playerNamesStrings);
+	scriptInterface.SetProperty(msg, "expectedHash", expectedHash);
+	scriptInterface.SetProperty(msg, "hash", hash);
+	scriptInterface.SetProperty(msg, "path_oos_dump", path.string8());
+	scriptInterface.SetProperty(msg, "path_replay", m_Replay.GetDirectory().string8());
+	m_NetClient.PushGuiMessage(msg);
 }
Index: source/simulation2/system/ReplayTurnManager.h
===================================================================
--- source/simulation2/system/ReplayTurnManager.h
+++ source/simulation2/system/ReplayTurnManager.h
@@ -41,8 +41,6 @@
 
 	void DoTurn(u32 turn);
 
-	void OnSyncError(u32 turn);
-
 	// Contains the commands of every player on each turn
 	std::map<u32, std::vector<std::pair<player_id_t, std::string>>> m_ReplayCommands;
 
Index: source/simulation2/system/ReplayTurnManager.cpp
===================================================================
--- source/simulation2/system/ReplayTurnManager.cpp
+++ source/simulation2/system/ReplayTurnManager.cpp
@@ -77,7 +77,11 @@
 	hash = Hexify(hash);
 
 	if (hash != expectedHash)
-		OnSyncError(turn);
+	{
+		m_HasSyncError = true;
+		LOGERROR("Replay out of sync on turn %d", turn);
+		g_GUI->SendEventToAll("ReplayOutOfSync");
+	}
 }
 
 void CReplayTurnManager::DoTurn(u32 turn)
@@ -100,16 +104,3 @@
 	if (turn == m_FinalTurn)
 		g_GUI->SendEventToAll("ReplayFinished");
 }
-
-void CReplayTurnManager::OnSyncError(u32 turn)
-{
-	m_HasSyncError = true;
-
-	std::stringstream msg;
-	msg << "Out of sync on turn " << turn << "\n\n" << "The current game state is different from the original game state.";
-
-	LOGERROR("%s", msg.str());
-
-	if (g_GUI)
-		g_GUI->DisplayMessageBox(600, 350, L"Sync error", wstring_from_utf8(msg.str()));
-}