This feels a lot like a bad install. Sometimes this occurs because of the state of the machine at the time of upgrade. A clean reinstall will usually cover it. Clean Lightroom Install Procedure Close Lightroom Restart the computer Use the Adobe Creative Cloud App to uninstall Lightroom Restart the computer Install Lightroom via the Creative Cloud App without launching any other programs. Restart the computer Launch Lightroom Wait 5 minutes
After a very frustrating afternoon trying to get Adobe Cloud to work! Never have I had to do so many restarts! ;o) But it is finally all done according to your instructions and the export is working again. I am delighted!