Thanks, Rainy! Your solution works, but one needs to shut Rainlendar completely before editing rainlendar.ini, not just refresh the app as usual. It would be great if you could add this as a normal option, so that completed tasks could be shown and hidden again in the to-do list - without having to perform this tedious shutdown-edit-relaunch procedure.
I had been excited about such an easy solution, but it doesn't work. I also tried adding the "TodoListHideCompleted=0" line under [OverriddenSettings/Shadow4/Day\ View]. This also didn't work, but I suppose that's where an additional line might belong, if there can be one now. Could you please tell me the correct syntax?