I tried all of the suggestions above (removing, reinstalling, restarting) and nothing worked. What finally worked was just starting a new return and then opening the file after that new return was started.
It "might" have something to do with having it stored on icloud and it had to re-download/re-open. When it was on-disk it had an "icloud" extension, which I think might be a compressed version and kept it from opening.
All just a guess ... but I finally got it open that way
