Hang on... Luma Updater also updates the boot.firm on CTRNAND, so if he can access the Homebrew Launcher, he can fix it.
@KleinesSinchen, does your n3DS boot at all with the latest Luma boot.firm on your SD root? If not, then we cannot help you fix it. If you can get your n3DS to boot, make sure that there is a working Wi-Fi connection setup on it, and if there is, follow these steps:
Put the Homebrew Launcher's boot.3dsx on your microSD root with Luma's boot.firm, create a '3ds' folder on your microSD root if it doesn't exist, download the latest release of Luma Updater from its GitHub page, copy the .3dsx file to the /3ds/ folder, boot your n3DS, open Health & Safety, press the L + Down (on dpad) + Select buttons to enter the Rosalina menu, go to 'Miscellaneous options > Switch the hb. title to the current app', then close and reopen H&S. Choose Luma Updater from the homebrew list, and use it to update both the microSD card's boot.firm AND the CTRNAND's boot.firm. This SHOULD fix your problems.
