Every few months, Rockstar Games releases a major update for GTA Online (e.g., The Contract, Drug Wars, Chop Shop). While these updates bring new cars and missions to the online component, they often break the single-player modding scene. The game's executable file changes, the version number bumps up (e.g., from .2545 to .2612), and the scripting hooks that mods rely on to function are severed.
If you are looking to run an older build, follow these community-standard precautions: lspdfr old version