Hi Petra,

It may be that Excel has hung and its background process is still running.

To check this, please ensure that all of your work has been saved, then do the following:

Press the Ctrl+Alt+Delete keys simultaneously, then select Task Manager from the list shown on the screen.

A window similar to the one below should appear.

Select Microsoft Excel, then click on End task.

If Microsoft Excel does not appear in this window, click on More details, then search for Microsoft Excel in the extended list displayed.

Run the add-in installer again after ensuring that Microsoft Excel no longer appears in the Task Manager.

Kind regards,
Ian