I installed mods and must have broken something. I uninstalled the mods, but the problem didn't go away, so I uninstalled and reinstalled the entire game, and it still displays the same error.

I have the Polish version of the game, and the message means something like this:

To load a saved game, you need the following content: Blood and Wine DLC, Heart of Stone DLC, Ciria's alternate appearance, and a few new quests.

test

Check your saved game files folder and remove the most recent entry (the one which was created just prior to your issue.)
You might want to keep the file in case it was not the factor which caused the loading problem.