I recently have found a way to boot sysUpdater with a higher success rate. I have tested this with my N3DS 10.3.0-28U and it worked in the first try. First, you have to use an entry point and install menuhax. Once that is done, download the attached sysUpdater build by @julian20 and extract it. Then place 'sysUpdater.3dsx' NOT 'sysUpdater.smdh' onto the root of your 3ds sd card. Now either back up the 'boot.3dsx' or rename it, as long as there is no longer a file with the name mentioned on your sd card. Now rename the 'sysUpdater.3dsx' that you placed on your sd card to 'boot.3dsx'. Next, you need to create a folder called updates on the root of your sd card. You then have to download the cias that you will be using to downgrade and place them in the updates folder you created (can't help you here). Now put your sd card in your 3ds and launch via menuhax. It should now work with a higher success rate then before. Please comment to let me know if this worked for you. Edit: This may also have a better chance for installing legit cias with FBI. I haven't tried this myself, so can someone confirm this? Use this guide (https://gbatemp.net/threads/how-to-install-legit-cia-files-above-9-3.407854/) and do the same thing with the FBI.3dsx file. So far there have been no confirmed reports, but still feel free to try. Edit 2: This does not work with increasing the success rate when installing legit CIA's with FBI. Oh well, it was worth the try. Edit 3: For those who have been getting garbled red letters or just simply haven't still been able to get this to work, try downloading @TuxSH build of sysUpdate which is attached to this post. Edit 4: I take no responsibility whatsoever if this bricks your device or anything else that may result from this.