WiFi Fixes It! Error Has Occurred. Hold down the Power Button
My sister's 3DS XL is just shy of 2 years old and about a few months out of its warranty period it would crash at any screen regardless if it was a game or just the Home Screen UI. I put it off to the side till I noticed it would not show the "Error Has Occurred. Hold down the Power Button to turn the power off" error message if the WiFi toggle was turned off, able to play games or do anything else for hours but the minute WiFi is back on it will error out. I first tried reseating the WiFi card as instructed by the guide but when that was no go I figure it wouldnt hurt trying another WiFi card instead of sending it to Nintendo for the price of practically a new 3DS.
The repair was straight forward, it also gives a change to clean out any debris underneath the backplate. The screw that is closest to the game cartridge would not fully come out however most likely with the thread being stuck in the plastic, but with force after all the other screws were out the back panel came off simple enough.
Installing the WiFi module itself was the easiest part, just make sure the antenna has enough slack for the new card to sit in properly.
The new WiFi card fixed the issue completely and a $15 dollar repair vs $130 Nintendo replacement is so much more worth it, and all my sis' extra content is completely safe too! No one thinks to backup until it's already too late.
If you're experiencing the "Error Has Occurred. Hold down the Power Button to turn the power off" try this and see if it helps you like it did for me.