I cut and moved the three files ilif01.1pe, ilif02.1pe and il_ind_schsa.pdf all dated 12/10/18 over to my desktop just in case I needed to restore them. When I restarted TurboTax with the files removed from their original locations, the problem was resolved.
Don't look for a prompt to download any updates, I never got one.
The il_ind_schsa.pdf file was a 2017 version of the Schedule SA form.
The other two files I examined using Notepad. Found loads of readable text among the jibberish, but no clear indication what they were supporting.
It would be nice to hear an official comment from TurboTax/Intuit about what these three files are about and how they're impacting the product's operation. Are they vital somehow? Why/how did the program work when these were removed?