When I try to define a Variables or constants, i end up in an infinite boot loop....

willkommen_text:
satz db 'sleeping_Dragon Kernel-Projekt V.-0.0.0-1',13,10,0
ret

can somebody tell what's wrong, and what i must do to prevent this behaviour?

are you expecting the string to be executable? the "ret" will not be executed if you fall into (or jump into) the "willkommen_text" address. the code will crash from trying to execute the string as code.