Well I didn't really expect it buuut... My new M3DS Real flashcart arrived today, and I did a backup of my SD card, formated it with a cluster size of 32kbyte and put everything on it again, including the NEW Sakura firmware. (I know some cheats won't work but I'm not the type for cheating anyway ^^) and guess what? It WORKS! Everything's perfectly fine, so I think my old one was either a fake, or it was broken... Anyway, I'd like to thank everyone who tried to help me out here.