Maybe those saves are from old games that will not play after the upgrade.

If you start a brand new game and then save, can you load that save?

If you want to play older, but still 0.70 Saved games, you will probably have to play using the stable branch, rather than the Test Branch.

added: It turns out that the versions we could download yesterday (Wed 5 Jan) have a problem loading a saved game if you are already playing the game. The only way is to exit the game to desktop and restart GC IV and then you can reload a Saved Game. Even trying to exit the game to Menu seems to lock up.

If you tried to Load the game while playing it, the game will lock up or do a crash to desktop, if it locks up use Windows Task Manager (Ctrl-Alt-Delete) to end the GC IV process. Once you reach the desktop, you can reload the game and then load the saved game file.