Hello I recently bought a 3DS and a R4 card. Yesterday when I used the R4 card it worked fine but today after I upgraded the 3DS to 2.1.0-4 it says "An error has occurred. Press and hold the POWER Button to turn the system off." I use a R4 3ds Pocket card. www.r4ipocket.com help
According their official site, their last update was done on , so you can't use it anymore since your 3DS is updated to the latest version. What you can do is buying another well supported flashcart like acekard 2i, supercard dstwo. Personally I recommend slickgate , they always test before shipping ,also if you buy the microsd together, they will help update the flashcart with the latest firmware for free.
should update your card to support V2.1.0-4 or choose another card. R4i gold can work on your 3DS without updating.