Open alpha testing of the 5.0 HD mod (online lobby).

As far as I understand, a save from the Olden Era cannot be completed with the Olden Era active, because the Olden Era crashes upon exiting the game.

This is incorrect. Moreover, a game started without the Olden Era can be continued with it active by loading the save.
The only thing that might not be immediately clear is that when playing with the Olden Era, you should always load the blue player's autosave. This is because the red player's saves (both autosaves and manual saves) do not save the blue player's turn (as if that turn hasn't happened yet).
You can, of course, load the red player's save, but then the blue player will have to repeat their turn.