I know this thread is older now but I was having the same issue and FOUND A SOLUTION after many failed attempts (using the 2019 Home & Business CD software on a Mac.)
When I went to import my 2018 return into the 2019 program it prompted me for my password for the 2018 return.
*Instead of typing in my password I selected "Forgot password" and answered the security question I had set up* The import time took longer this time and sure enough the 2018 info was there!
I immediately tested this method by starting a new return, importing the old return and typing in the password as normal (NOT "forgot password) and just as before it did NOT work this way. (Said it imported everything but nothing showed up.)
Not sure if this is the end-all solution, but I hope this helps other users who are just starting their returns (like me) as well as the developers who might be able to take this as a clue to the underlying issue.
@8tgibson @Anonymous_ @JamesG1 @hdfxrs @Neil10
