You're supposed to create the backup using Bootstrap Recovery/Clockwork Mod
You're either going to have to find a rom of Froyo 2.2 or .sbf I think
If you didn't make a backup through Clockwork Recovery, you're going to have to .sbf...from my own experiences, I've had to .sbf to get off Rubix without restoring a backup (I had a backup, but I wanted to start fresh rather than using the backup) are you running 2.3.15? or did you update to the link? what version of Rubix are you running?