Index: binaries/data/mods/public/gui/gamesetup/gamesetup.js
===================================================================
--- binaries/data/mods/public/gui/gamesetup/gamesetup.js
+++ binaries/data/mods/public/gui/gamesetup/gamesetup.js
@@ -262,6 +262,11 @@
 var g_GameStarted = false;
 
 /**
+ * Whether the player has been warned already that rejoining in multiplayer mode is not supported.
+ */
+var g_AIMultiplayerWarning = true;
+
+/**
  * Selectable options (player, AI, unassigned) in the player assignment dropdowns and
  * their colorized, textual representation.
  */
@@ -863,6 +868,30 @@
 		"hidden": () =>
 			!g_PlayerAssignments[Engine.GetPlayerGUID()] ||
 			g_PlayerAssignments[Engine.GetPlayerGUID()].player == -1 && !g_IsController,
+		"onPress": () => function() {
+
+			let callback = g_IsController ? launchGame : toggleReady;
+
+			if (
+			    !g_IsNetworked ||
+			    !g_AIMultiplayerWarning ||
+			    !g_IsController && g_IsReady == 2 ||
+			    g_GameAttributes.settings.PlayerData.every(pData => !pData.AI))
+			{
+				callback();
+				return;
+			}
+
+			g_AIMultiplayerWarning = false;
+			messageBox(
+				400,
+				200,
+				translate("Rejoining multiplayer games with AI players is not supported!") + " " +
+					translate("Do you want to continue?"),
+				translate("WARNING"),
+				[translate("No"), translate("Yes")],
+				[null, callback]);
+		}
 	},
 	"civResetButton": {
 		"hidden": () => g_GameAttributes.mapType == "scenario" || !g_IsController,
Index: binaries/data/mods/public/gui/gamesetup/gamesetup.xml
===================================================================
--- binaries/data/mods/public/gui/gamesetup/gamesetup.xml
+++ binaries/data/mods/public/gui/gamesetup/gamesetup.xml
@@ -250,14 +250,7 @@
 				style="StoneButton"
 				size="100%-164 100%-52 100%-24 100%-24"
 				tooltip_style="onscreenToolTip"
-			>
-				<action on="Press">
-					if (g_IsController)
-						launchGame();
-					else
-						toggleReady();
-				</action>
-			</object>
+			/>
 
 			<!-- Cancel Button -->
 			<object