Thanks for your reply.
The custom tab became not visible.
The XML code is saved in this specific workbook.
But I have been able to restore it.
I had changed something in the XML code in Custom UI Editor and apparently this was not accepted by excel.
It was however correctly formed in Custom UI Editor.
I added the physical address of the ribbon, a number that is generated each time.
I did this because I learned this during the course.
Actually, I do not quite understand how this works. Is it possible to send me more information about this?
Thanks in advance.