I'll attach the save but there are quite a few units on the board, I've reproduced this 2x now.
I have a Battleship army which is escorting a Great Admiral. I upgraded the Battleship to Missile Cruiser and continued progressing through each unit and city that needed an order. I got to the point where the bottom right corner of the screen said I had a unit which needed to be moved but it wouldn't select the unit. I could not progress to the next turn.
I guessed that it might be that Admiral, I manually selected the Admiral and slept him and it let me progress.
Patch: 1.0.0.26 (221715)