The issue has been identified and a workaround is being worked on. It’s not a Frostfeast issue per se, rather the engine seems to dislike a trace the new firepit component does… and apparently, only on some systems/some saves.
Currently, I suspect that Windows 7 machines are unable to load saves that contain a firepit, or chimney, that has no fuel in it. Before I get around to fix it, playing without any deployed firepits or chimneys should allow you to save and load said saves properly. Note that the goblin firepit is affected by this, too.