1. What happened
When moving the mouse pointer from Collab Toolbar or Services or Collab History windows to 'Scene View' error is thrown to console:

InvalidOperationException: Stack empty.
System.Collections.Stack.Pop () (at /Users/builduser/buildslave/mono/build/mcs/class/referencesource/mscorlib/system/collections/stack.cs:170)
UnityEngine.GUILayoutUtility.EndLayoutGroup () (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:323)
UnityEditor.HostView.EndOffsetArea () (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:200)
UnityEditor.HostView.InvokeOnGUI (UnityEngine.Rect onGUIPosition) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:248)
UnityEditor.DockArea.OldOnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:407)
...

or sometimes the exception is a bit different:
InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Pop () (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections/Stack.cs:329)
UnityEngine.GUILayoutUtility.EndLayoutGroup () (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:323)
UnityEditor.HostView.EndOffsetArea () (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:200)
UnityEditor.HostView.InvokeOnGUI (Rect onGUIPosition) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:248)
UnityEditor.DockArea.OldOnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:407)
...

But repro steps are the same

2. How we can reproduce it using the example you attached
- Have project connected to Collab
- Open either of these: 'Collab Toolbar', 'Services', 'Collab History'
- Place it in Editor layout so it'll be next to Scene View
- Move mouse pointer over one of opened collab windows
- Wait for 2-3 seconds
- Move mouse pointer to Scene View
---> Error appears in Console

Note: Sometimes it is not 100% reproducible. But I'm getting it most of the time. Might need to open some of the other 'offending' windows and switch between those and repeat the repro movement. But the error appears after few tries.

Regression from Version 5.6.0p1 (6c0210300415)