That would work, but not needed I believe as one of the by-products of rebooting will be the closure of your File Explorer window(s).Peter_SSs,
Thank you. I rebooted and it seems to be working now.
OK, thanks for the additional information. The issue seems to be something local (I have it too) as not all users can reproduce the problem by following identical steps. However, as yet we have not been able to nail down exactly what is causing it.It turns out I had both Microsoft Edge and FireFox open. I first tried closing both of them plus Excel (365 on Windows 10) and all other programs, but still had the issue when I opened Excel back up. That's why I did a re-boot. Maybe something was in memory that didn't clear until the re-boot. Thanks to you and Mark858 for your help.