[moderation: please don't follow these instruction. look further down for the ctrl-click solution]
I solved the problem.
First, I had to add the following command line in Terminal: sudo spctl --master-disable. This gave me the option on the General tab of Security & Privacy to select "Anywhere" in the "Allow apps downloaded from: section. This option has been removed from the latest versions of MACOS.
Second, on the Privacy tab, I unchecked Scribus in the "Allow the apps below to control your computer".
Once I did this and rebooted, Scribus opens like it did before.
Problem solved!