Errors when using GUILayout in ModalWindow

I am trying to show a modal window and fill it using GUILayout. I have done this in another script without any problems, and can't see what I'm doing differently here.

When I use "Begin/End Horizontal", I get this error: "ArgumentException: Getting control 1's position in a group with only 1 controls when doing Repaint". I have googled the error and other answers don't seem to be related.

 void OnGUI() {        
     if (UserLoggingIn) {
         GUI.ModalWindow(1, loginWindowRect, LoginWindow, "Login");
     }
 }
 
 void LoginWindow(int windowID) {
     GUILayout.BeginHorizontal();
     if (GUILayout.Button("Cancel")) {
         Debug.Log("Cancel");
     }
     if (GUILayout.Button("Login")) {
         Debug.Log("Login");
     }        
     GUILayout.EndHorizontal();
 }

If I remove the Horizontal lines, the error clears, but the buttons do not respond. I can highlight the buttons, but clicking them does nothing.

 void LoginWindow(int windowID) {
     if (GUILayout.Button("Cancel")) {
         Debug.Log("Cancel");
     }
     if (GUILayout.Button("Login")) {
         Debug.Log("Login");
     }        
 }

I'm wondering if another script could be interfering, but can't see how. Any ideas? The property UserLoggingIn is not being changed after initialisation.