Odd. Odd. Odd. 🙂
I got frustrated and decided to restore a back up of everything that I had made earlier (before this process began) from within SI. I deleted the tables in tsp_invoices and then imported the back up. Suddenly, as if by magic, I can Manage Products in Version: 2019.1.1 . There’s still the issue with the currency symbols to deal with but I want to set up a database copy to test on.
The error occurred when I was importing the actual data. I suspect it may have been my dump of the data ie I think I left “structure” ticked (as well as data.) Do you think this may have caused the issue?
Once again, many thanks for your patience and help.