After having played the game for about a week without any problems, today suddenly none of my saves work. Everytime I try to load them I get "Unable to load save, file contains invalid data".

How can I fix this? I obviously can't play a game where I can't even save my progress..

I'd really like some help with this, I can't play the game knowing my saves will just break after a few days.

Not only are my saves broken, but the game looks like it completely reset. All the scenarios I had unlocked are locked again.

Could someone at least tell me how to prevent this from happening in the future?

Have you been using a trainer?

When RCT loads and saves games it uses a special routine that 'sum checks' the file. The routine knows if you have been using a trainer, and refuses to load the file, and puts up the invalid data message.

Nope, I never used a trainer or any cheat for that matter.