I took an extended break from my campaign in SMAC, and when I came back and tried to load my ongoing game, the game told me that it was a "password-protected multiplayer session" and I wasn't allowed to access it. I haven't played SMAC multiplayer in my life and this campaign was definitely neither multiplayer nor password-protected.

Is there any way to inform the game that it is mistaken and allow me to resume my game?

According to this old thread at Civfanatics: https://forums.civfanatics.com/threads/cannot-load-game.431699/

It can happen if the savegame was made with the expansion (SMAX) but you tried to load it on vanilla (SMAC). Could that be the case?

...huh. Yes, it very much could be. But I only have one executable, so how do I launch the expansion instead of the base game?
edit: nevermind, I found it under the right-click menu. Thank you :).

I'm running into this issue as well, it's trying to launch the game as hotseat multiplayer? Even when I just started a fresh install and go straight to the generic single player game setup.