Skype for Business unable to set Call Forwarding

Scenario: Call forwarding used to work fine.

Fault:  Attempting to set the call forwarding fails.  The Skype client briefly flashes but does not explain the reason for the failure.

Solution:  User was logged into two workstations with two Skype for Business apps running.  Exiting the second Skype for Business instance allowed the first one to correctly accept a call forwarding state.

Addin LoadBehavior values

https://docs.microsoft.com/en-us/visualstudio/vsto/registry-entries-for-vsto-add-ins?view=vs-2019

LoadBehavior values
The LoadBehavior entry under the Root\Software\Microsoft\Office\application name\Addins\add-in ID key contains a bitwise combination of values that specify the run time behavior of the VSTO Add-in. The lowest order bit (values 0 and 1) indicates whether the VSTO Add-in is currently unloaded or loaded. Other bits indicate when the application attempts to load the VSTO Add-in.

Typically, the LoadBehavior entry is intended to be set to 0, 3, or 16 (in decimal) when the VSTO Add-in is installed on end-user computers. By default, Visual Studio sets the LoadBehavior entry of your VSTO Add-in to 3 when you build or publish it.

The following table lists all the possible values of the LoadBehavior entry. Some descriptions in this table refer to loading a VSTO Add-in manually or programmatically. To load a VSTO Add-in manually, select the check box next to the VSTO Add-in in the COM Add-Ins dialog box in the application. To load a VSTO Add-in programmatically, set the Connect property of the Microsoft.Office.Core.COMAddIn object that represents the VSTO Add-in to true.

Value (in decimal) VSTO Add-in status VSTO Add-in load behavior Description
0 Unloaded Do not load automatically The application never tries to load the VSTO Add-in automatically. The user can try to manually load the VSTO Add-in, or the VSTO Add-in can be loaded programmatically.

If the VSTO Add-in is successfully loaded, the LoadBehavior value remains 0, but the status of the VSTO Add-in in the COM Add-ins dialog box is updated to indicate that the VSTO Add-in is loaded.
1 Loaded Do not load automatically The application never tries to load the VSTO Add-in automatically. The user can try to manually load the VSTO Add-in, or the VSTO Add-in can be loaded programmatically.

Although the COM Add-ins dialog box indicates that the VSTO Add-in is loaded after the application starts, the VSTO Add-in isn't loaded until it is loaded manually or programmatically.

If the application successfully loads the VSTO Add-in, the LoadBehavior value changes to 0, and remains at 0 after the application closes.
2 Unloaded Load at startup The application does not try to load the VSTO Add-in automatically. The user can try to manually load the VSTO Add-in, or the VSTO Add-in can be loaded programmatically.

If the application successfully loads the VSTO Add-in, the LoadBehavior value changes to 3, and remains at 3 after the application closes.
3 Loaded Load at startup The application tries to load the VSTO Add-in when the application starts. This is the default value when you build or publish a VSTO Add-in in Visual Studio.

If the application successfully loads the VSTO Add-in, the LoadBehavior value remains 3. If an error occurs when loading the VSTO Add-in, the LoadBehavior value changes to 2, and remains at 2 after the application closes.
8 Unloaded Load on demand The application does not try to load the VSTO Add-in automatically. The user can try to manually load the VSTO Add-in, or the VSTO Add-in can be loaded programmatically.

If the application successfully loads the VSTO Add-in, the LoadBehavior value changes to 9.
9 Loaded Load on demand The VSTO Add-in will be loaded only when the application requires it, such as when a user clicks a UI element that uses functionality in the VSTO Add-in (for example, a custom button in the Ribbon).

If the application successfully loads the VSTO Add-in, the LoadBehavior value remains 9, but the status of the VSTO Add-in in the COM Add-ins dialog box is updated to indicate that the VSTO Add-in is currently loaded. If an error occurs when loading the VSTO Add-in, the LoadBehavior value changes to 8.
16 Loaded Load first time, then load on demand Set this value if you want your VSTO Add-in to be loaded on demand. The application loads the VSTO Add-in when the user runs the application for the first time. The next time the user runs the application, the application loads any UI elements that are defined by the VSTO Add-in, but the VSTO Add-in is not loaded until the user clicks a UI element that is associated with the VSTO Add-in.

When the application successfully loads the VSTO Add-in for the first time, the LoadBehavior value remains 16 while the VSTO Add-in is loaded. After the application closes, the LoadBehavior value changes to 9.