Instructions
Restart your DSi and try booting up the game without codes. If the game boots up when codes are not activated, then it is probably an individual code causing the problem. It's tedious but you can try activating each cheat one by one until you identify the code causing the error and remove it from your Action Replay database.
Activate a fewer number of cheats and try to load the game up again. Sometimes too many codes are too much for the game's software to handle. Try only using the cheats you really want and try loading the game again. If this does not help, try one more time with only one or two codes activated. If you are still having problems with two or less codes activated, this isn't the reason for your error.
Clean your Action Replay device and DSi video game unit extensively. Using a Q-Tip moistened with rubbing alcohol is effective. You can also trying blowing into your devices to dislodge any dust that may have accumulated over time. If you have had your DSi for awhile or you purchased your game used, this is probably the reason for the error.