Modified versions of the game aren't compatible with saves from the unmodified game or from other mods, and it's not fixable. It's possible for mods to block incompatible saves from being loaded but this hasn't been done in A-spec yet as far as I'm aware.
For the technical reasons why they're incompatible: the savegame just records that your car has, for example, the 115th engine in the game data, the 320th turbo kit, the 115th drivetrain, etc. This works fine in the original game as the car parts will always be the same. When mods add or remove parts, the save is now pointing at the wrong parts, because if a new engine was added higher up the list the correct engine for the car is now the 116th instead of the 115th in the list (for example). In theory it could be possible to edit the save with a hex editor to replace every single part on every single car in the garage with the correct one in that mod, but you would probably need to create or modify mod tools just to figure out what the right values would be, and it would take a very long time.
