I can donate a valid ctcert from a system ill never use again(half broken old 3dsxl).And yeah i just tried renaming again just to check if i was wrong and nope.So i suggest maybe using an injected srl.nds from a CFW 3Ds instead of just a renamed sudoku_v0.app.
Just confirming something but renaming sudoku_v0.app to srl.nds actually didnt work for me i had to inject the .app using a CFW 3ds and dump the game to get a valid srl.nds.
With just renaming the sudoku_v0.app to srl.nds the target 3Ds wouldnt allow me to import the edited game but it worked...