If you have a Nintendo 3DS or 2DS console, you probably know CFW. Custom firmware is very useful for many reasons.
Installing a CFW can be a bit complicated. An obstacle that I personally encountered during a CFW installation was the infamous ARM 11 error.
I was curious to see what the newly launched Luma3DS 10.0 CFW offered, and I was trying to get the homemade launcher to work through Rosalina's menu.
Once I left the Download Play application, and opened it again to open the homebrew launcher. This step, which is a common part of the installation, should have worked, but it did not.
What did not work for me
Let me save you some time and tell you what you should not do. Naturally when I thought the application did not work, and tried a different one.
A commonly recommended solution is to use the Health and Information application, which is preloaded on the Nintendo 3DS console. But when I tried, the console gave me the same error.
I also tested the options in the Luma configuration menu, which includes the Game patch, and disabled ARM 11 errors, without success.
How to fix an exception error on the Nintendo 3DS console
Start by turning off the console.
Connect the SD card to your computer and check the following folders:
3DS folder – This should contain the ctr-no-timeoffset.3dsx and FBI.3dsx folder. Do not put your boot.3dsx file here.
The root folder of the SD card – You must have the file boot.3dsx.
If any of them are missing, download them and copy them to their corresponding folders.
Open the Download Play application. Once you show the 2 options, try to launch the Rosalina menu using the key combo: Left shoulder + Down + Select. Set the title Change the title of the current application »and then press B twice to exit the menu.
Close the Download Play application and reopen it. The home launcher with the CTR and FBI options should appear on the screen.
In my case, I had to download the boot.3dsx file again and put it at the root of the SD card. I could also confirm that this was the solution, removing the boot.3dsx file and I was able to produce the error screen successfully.
The only other files involved during the process are the CTR and the FBI. Therefore, you may want to try replacing them too, in case the solution does not work for you.