That's completely normal. All mods that modify the game engine will crash when loading savegames not made with the mod, and savegames made with such mods running will crash when playing without the mod.

A savegame is a memory dump of the current state of the running program, and a modded game will have things organised differently in its memory, inevitably making it incompatible. I don't think any mods bother making a specific compatibility system to correctly load vanilla savegames. It'd be pretty complex to pull off, basically requiring them to keep original copies of pretty much all game code just for that purpose.

So simply don't load old savegames. Load missions from the Mission Select menu. You should still be able to make new savegames just fine, and load those new savegames made with the mod.

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Oddly enough, the original game had a system to prevent these crashes: it did some simple checks on the lengths of the game objects used in savegames, and baked that into a checksum value that was saved in the savegames. With that, it could determine which savegames were incompatible with its own version, and it would mark such savegames as "[old]" in the "Load game" menu. But it seems the remaster did not preserve that system for some reason.