Cheats will be disabled for GBA for about the next week. Any cheats you have applied should still work. It's because I have to update the implementation as I switched from standalone mGBA to a retroarch core version. 

The good news is that I will be implementing cheats in a way that they will work across all consoles and I will also be supplying a cheats database so you will be able to just select the cheats you want to use from a drop down. No more codes :)