Firstly, I read this using Google translate.
The contents of the KOBOReader partition can be deleted and a setup will be triggered when the device is restarted. This will recreated the contents of the .kobo directory. But, it does not sound like the device is reaching this point.
I think the problem will be with KSM. I did not think there was a version of this compatible with the recent firmware versions and the Clara HD. Installing it changes the boot process and if it fails, would behave like you are saying. At this point, you need probably need to try the manual factory reset as described on
https://help.kobo.com/hc/en-us/articles/360017605314.
The only other way to fix this is probably to restore an image to the SD card. I have sent you a PM with the link to the image as I suspect you will need it.