You must flash it with the new firmware using an older DS. Your DS has updated, so you must also.
Are you 100% sure? So I put the latest kernel and update it to what kernel from a DS? It's already on the latest firmware!
Kernel =!= Firmware
Firmware: The basic OS of the flash card. You can see it working best if you have no menu files on your Micro SD card. Basically, it's the stuff that tells the card what to do with compatible menu files and, if it's a DSi/3DS compatible card, how to bypass the anti-flash card protection on a DSi or 3DS.
Kernel: The visual OS of the flash card. It tells the firmware of the card what to output when not in a game and how things should work.
Take this as an example: DOS may be the firmware of a computer, but Windows is the kernel. Give or take some complicated things I don't understand.
With DSi and 3DS compatible flash cards, you need to update both the firmware and the kernel for the best results.
However, if your DSi or 3DS is already updated to a version not supported by your card, you must obtain an older DS (either Phat or Lite) and use a special update file that updates the firmware to support bypassing of the newer DSi or 3DS anti-flash card protection. Some cards may not have updates to a newer version at the point you may need it, so if your card doesn't have a firmware update then you'll have to wait (or, if there's been a very lengthy amount of time since the last firmware update, you may have to buy a new card).
I hope I've explained this well enough.