The mistake is usually hard to make out. I was able to fix the devices with software errors submitted to me as follows.
# 01
Boot to Recovery Mode / A + R + L + Up (DPad) and update the device.
-> then the device should start regularly.
# 02
if recovery mode does not work, as it may be corrupted too, I've got a recovery like this:
- CRT transfer via Decrypt9
- Install Luma3DS in SYSNAND (CRTNAND).
- start 2DS / 3DS without SD card (since Luma3DS is installed in the CRTNAND)
- either the 2DS/3DS starts normally
- or wait 30 - 60 minutes if necessary; In some cases, it may take a while for the home menu to be restored.
- then you can also insert the SD card and best directly change the design (from white to black), so that the home menu is also updated on the SD card.