hmm... well, sorry my reply wasnt so speedy this time, but you can try:
1) leaving the .3dsx file for sysupdater in the 3ds folder alone, not within a subfolder. i was able to launch it as a lone file, but i didnt use that method. (since nobody tried this, you might not feel safe doing it. that's fine, i'm not sure it would do much anyway. when i was using ironhax the .3dsx in a subfolder showed corrupted text, while the .3dsx alone in the 3ds folder showed regular text. neither responded to me pressing any buttons.)
2) turning off network. before i turned off the wifi on my 3ds i couldnt really get the file to start, not sure if they had anything to do with each other.
3) install oothax. another entrypoint. the homebrew starter kit had a .3dsx file just for this.
4) install tubehax. the more entrypoints the better. ( look at smealums github for more info )
5) there is no 5
6) try reformatting the sd card. maybe your problem has something to do with the sd card or the copied file was a bit messed up.
i ordered them in the order i would try them, easiest, to hardest/least likely to do something. also, double check that everything is where it should be. .3dsx files are in the 3ds folder, or in a subfolder in the 3ds folder, the update cias are for the correct FW version, hardware model, and region, and place in a folder titles 'updates' on the root of the sd card.
Good Luck.
PS since nobody tested the lone .3dsx file other than what i did, after you try the .3dsx alone, go back to using the .3dsx in a subfolder, because you might as well try it normally if you install another entry point.
Good luck again.
Always touching 
] brb
