GUI.Window function not called anymore

I have some weird problem. I don't use GUI stuff much in my current game (or just stick to the basics), but there was one place where I used GUI.Window to open a confirmation window. Back then when I wrote that part of the GUI everything worked.

I clicked on a button to reset the current games progress and a window opened with a confirmation. Now, a few months later it doesn't seem to work and nothing was changed there in code.

     if(GUI.Button(new Rect(Screen.width/2-140, Screen.height/2+60, 290, 50),"Clear progress")) {
         bResetStatsDialog = true;
     }

     if(bResetStatsDialog) {
         // Enable GUI elements
         GUI.enabled = true;

         GUI.Window(1, new Rect(Screen.width/2-200,Screen.height/2-100,400,200), DrawConfirmationWindow, "Confirmation Dialog");
     }
 }
 
 ...
 
 void DrawConfirmationWindow(int windowId) {
     GUI.DragWindow();
     GUI.BringWindowToFront(windowId);
     Rect windowRect = new Rect(30,70,380,200);

     // Display label text
     GUI.Label(windowRect,"Do you want to reset the current game state?");
     if(PlaySound(GUI.Button(new Rect(20, 140,100,40),"Yes"))) {
         // remove all saved settings
         PlayerPrefs.DeleteAll();

         // close window
         bResetStatsDialog = false;
     }
     if(PlaySound(GUI.Button(new Rect(250, 140,100,40),"No"))) {
         // close window
         bResetStatsDialog = false;
     }
 }

Once I hit the 'clear progress' button, the GUI.Window option is called on every frame correctly. However, the callback (GUI.Window's "func" parameter, here DrawConfirmationWindow) function is never called and I have no idea what's causing it.

Did anything changed with the GUI stuff in the most recent Unity version? (3.4/3.4.1)? I could swear it was working with 3.3 (with 3.2 I'm 100% certain). Or is it some kind of bug which was introduced with Unity 3.4?